“150”, Aptitude Test Questions and Answers for Statistician – National Bureau of Statistics (NBS).
ABSTRACT
This book contains 150 multiple-choice questions and answers designed to help candidates prepare for
the Statistician II aptitude test conducted through Tanzania's Public
Service Recruitment Secretariat (PSRS) for the National Bureau of Statistics
(NBS). The questions reflect the analytical and competency-based style
commonly used in government recruitment examinations and cover key areas such
as official statistics, survey methodology, sampling, data quality, statistical
analysis, reporting, ethics, confidentiality, demographic and economic
indicators, and evidence-based decision-making. Each question is accompanied by
the correct answer and a detailed rationale to strengthen understanding,
improve problem-solving skills, and build confidence for the actual
examination.
Prepared by: Statistician
Compiled by Statistician
Professionals stationed in Dar-es-salaam.
0628729934.
Date: July 20, 2026
Dear applicants,
This collection of questions and answers
has been prepared to help all of you to understand the key areas tested during
the interview. The goal is to provide a useful, and practical study guide so
you can all perform confidently and fairly in the selection process. I wish you
the best of luck, and may this resource support you in achieving success!
Warm regards,
Johnson Yesaya Mgelwa
For Personal Use by Applicants Preparing
for Statistician – National Bureau of Standards (NBS).
ALL QUESTIONS TOGETHER.
Question 1
During a nationwide household
survey, field supervisors discover that interviewers in one region consistently
skip questions they consider "not applicable" without following the
questionnaire instructions. If left uncorrected, what is the MOST serious
consequence?
A. Sampling variability increases
beyond planned levels. B. Measurement error becomes systematic across affected
records. C. Random error is equally distributed among all observations. D.
Sampling weights automatically become statistically invalid.
Answer: B. Measurement error
becomes systematic across affected records.
Rationale: When interviewers deliberately ignore
questionnaire instructions, they introduce systematic measurement error because
the collected responses no longer reflect the intended survey design. Unlike
random error, systematic error creates consistent bias that cannot simply be
reduced by increasing sample size. Sampling variability relates to the
selection of respondents rather than interviewer behavior, and sampling weights
do not automatically become invalid solely because interviewers skipped
questions. Maintaining interviewer compliance with survey protocols is
therefore one of the most important quality control responsibilities during
official statistical data collection.
Question 2
An NBS statistician is designing
a survey questionnaire intended for respondents with different educational
backgrounds across Tanzania. Which principle is MOST important for improving
response accuracy?
A. Include several technical
statistical terms to increase precision. B. Increase the number of questions
measuring the same characteristic. C. Use identical wording from previous
international questionnaires regardless of context. D. Arrange questions using
language that respondents easily understand.
Answer: D. Arrange questions
using language that respondents easily understand.
Rationale: The primary objective of questionnaire
design is to obtain accurate and reliable information from respondents.
Questions should therefore use clear, simple, and culturally appropriate
language that minimizes misunderstanding regardless of respondents' education
levels. Excessive technical language may confuse respondents, while merely
repeating international wording without adaptation can reduce validity. Adding
unnecessary duplicate questions may increase respondent fatigue rather than
improve data quality.
Question 3
A survey intends to estimate
average household income for every region of Tanzania. The statistician expects
substantial income differences among regions. Which sampling technique is MOST
appropriate?
A. Convenience sampling. B.
Systematic sampling. C. Stratified random sampling. D. Snowball sampling.
Answer: C. Stratified random
sampling.
Rationale: Stratified random sampling divides the
population into relatively homogeneous groups before selecting samples from
each group. Since household income is expected to vary considerably between
regions, stratification ensures that every region is adequately represented
while improving the precision of estimates. Convenience and snowball sampling
are non-probability methods unsuitable for official statistics, while
systematic sampling alone may not guarantee balanced regional representation.
Question 4
Before launching a national
survey, the questionnaire is administered to a small number of respondents
under conditions similar to the actual survey. What is the PRIMARY purpose of
this exercise?
A. Produce preliminary national
estimates for publication. B. Replace interviewer training with practical
experience. C. Determine the final sample size for every region. D. Identify
weaknesses in the questionnaire before full implementation.
Answer: D. Identify weaknesses
in the questionnaire before full implementation.
Rationale: A pilot survey is conducted to evaluate
questionnaire clarity, sequencing, respondent understanding, interview
duration, logistical arrangements, and data collection procedures before the
actual survey begins. Identifying these weaknesses early allows corrections
that improve data quality. Pilot surveys are not intended to generate official
national estimates, replace interviewer training, or determine the final sample
allocation.
Question 5
During data editing, two
variables indicate that a respondent is recorded as being both 12 years old and
married for 20 years. What should the statistician conclude FIRST?
A. The record contains an
internal inconsistency requiring verification. B. The respondent intentionally
provided false information. C. The questionnaire should immediately be
discarded from analysis. D. The coding manual must contain incorrect
classifications.
Answer: A. The record contains
an internal inconsistency requiring verification.
Rationale: Data editing begins by identifying
inconsistencies that violate logical relationships among variables. A
respondent aged 12 cannot realistically have been married for 20 years,
indicating that one or more recorded values require verification. The statistician
should first investigate possible recording, coding, or entry errors before
assuming respondent dishonesty or discarding the questionnaire altogether.
Question 6
An official statistical report is
scheduled for public release next week. Before dissemination, which principle
is MOST critical for maintaining public confidence?
A. Publish the report before
completing data validation. B. Allow selected institutions early access without
approval. C. Ensure the reported statistics have undergone quality assurance
procedures. D. Modify results that appear inconsistent with public
expectations.
Answer: C. Ensure the reported
statistics have undergone quality assurance procedures.
Rationale: Official statistics must be accurate,
reliable, and credible. Comprehensive quality assurance—including validation,
consistency checks, and methodological review—helps ensure that published
statistics accurately represent the collected data. Publishing incomplete
results, providing unauthorized early access, or adjusting findings to match
expectations would undermine the integrity and credibility of official
statistics.
Question 7
A field supervisor notices that
one interviewer consistently completes interviews in half the expected time
while producing unusually few missing responses. Which action is MOST
appropriate?
A. Accept the interviews because
completeness appears satisfactory. B. Increase the interviewer's daily workload
immediately. C. Conduct targeted quality verification of the completed
interviews. D. Replace the interviewer without reviewing collected
questionnaires.
Answer: C. Conduct targeted
quality verification of the completed interviews.
Rationale: Unusually short interview durations
combined with exceptionally complete questionnaires may indicate fabricated
data or failure to follow interview procedures. Rather than assuming excellent
performance or immediately dismissing the interviewer, supervisors should
conduct verification through re-interviews, spot checks, or review of completed
questionnaires to determine whether quality standards were maintained.
Question 8
A statistician calculates the
average monthly expenditure of households, but a few extremely wealthy
households substantially increase the average. Which measure would BEST
represent the typical household?
A. Median. B. Mean. C. Variance. D.
Standard deviation.
Answer: A. Median.
Rationale: The median is resistant to extreme values
and provides a better measure of central tendency when data are highly skewed.
Household expenditure often contains outliers because a small number of
households spend much more than the majority. In such cases, the median better
reflects the typical household than the arithmetic mean.
Question 9
A survey achieves an overall
response rate of only 58%, with non-response concentrated among urban
households. What is the GREATEST concern?
A. Sampling errors automatically
become zero. B. The survey may suffer from non-response bias. C. The sample
size always becomes statistically adequate. D. Measurement errors are
completely eliminated.
Answer: B. The survey may
suffer from non-response bias.
Rationale: When certain groups respond less
frequently than others, the resulting data may no longer represent the target
population accurately. If urban households differ systematically from
respondents, estimates may become biased even if the original sample was
scientifically selected. High response rates and appropriate follow-up
procedures are therefore essential components of official statistical quality.
Question 10
Which characteristic MOST
distinguishes official statistics from unofficial statistical information?
A. They are always produced using
the largest possible sample. B. They are generated according to recognized
statistical standards and legal requirements. C. They must only be produced by
universities. D. They are published only after international approval.
Answer: B. They are generated
according to recognized statistical standards and legal requirements.
Rationale: Official statistics are produced within
an established legal and institutional framework using recognized statistical
methodologies, quality standards, and professional independence. Their
credibility depends on adherence to these principles rather than simply using
large samples or obtaining approval from external organizations.
Question 11
An NBS team is selecting
Enumeration Areas (EAs) from every district before households are selected
within each EA. This approach is BEST described as:
A. Simple random sampling. B.
Stratified sampling. C. Systematic sampling. D. Multistage sampling.
Answer: D. Multistage
sampling.
Rationale: Multistage sampling involves selecting
samples through successive stages rather than choosing all final units
directly. In large national surveys, Enumeration Areas are often selected
first, followed by households within those areas. This approach reduces field
costs while maintaining probability sampling principles and is widely used in
official household surveys conducted by national statistical offices.
Question 12
While reviewing completed
questionnaires, a statistician notices that different interviewers applied
different interpretations of the same question because the interview manual was
unclear. Which aspect of data quality is MOST directly affected?
A. Comparability of responses. B.
Confidentiality of respondents. C. Timeliness of publication. D. Accessibility
of statistical reports.
Answer: A. Comparability of
responses.
Rationale: Ambiguous instructions cause interviewers
to apply different interpretations when collecting information. As a result,
responses collected from different respondents are no longer measured
consistently, reducing comparability across observations. Official statistics
require standardized data collection procedures so that every respondent is
measured using the same concepts and definitions.
Question 13
A statistician wishes to compare
employment rates between regions that have substantially different population
sizes. Which approach provides the MOST meaningful comparison?
A. Compare the total number of
employed persons. B. Compare the number of households surveyed. C. Compare
employment percentages or rates. D. Compare the total questionnaires completed.
Answer: C. Compare employment
percentages or rates.
Rationale: Comparing absolute numbers between
populations of different sizes can be misleading because larger populations
naturally produce larger totals. Employment rates standardize the measure
relative to the population size, allowing meaningful comparisons across regions
regardless of differences in population.
Question 14
During fieldwork, one interviewer
begins replacing unavailable households with nearby households of his own
choice without authorization. What statistical principle has MOST likely been
violated?
A. Data confidentiality. B.
Probability-based sample selection. C. Statistical disclosure control. D. Data
dissemination policy.
Answer: B. Probability-based
sample selection.
Rationale: Probability sampling requires every
selected unit to be chosen according to the approved sample design.
Unauthorized substitution introduces interviewer judgment into the selection
process, potentially producing selection bias and reducing the representativeness
of survey estimates. Replacement procedures, if permitted, must follow
established statistical protocols.
Question 15
A report indicates that the
average household size is 4.8 persons. Which statement is the BEST
interpretation?
A. Every household contains
exactly five members. B. Nearly half the households contain five members. C.
The calculated mean household size equals 4.8 persons. D. Household size cannot
be expressed using decimals.
Answer: C. The calculated mean
household size equals 4.8 persons.
Rationale: A mean is an arithmetic average and does
not imply that every individual observation equals that value. Household size
may vary widely, and an average of 4.8 simply summarizes the overall
distribution. Decimal values are entirely acceptable for averages, even when
the original variable consists of whole numbers.
Question 16
Which action would MOST
effectively reduce interviewer bias during official data collection?
A. Allow interviewers to explain
questions differently whenever necessary. B. Assign interviewers only to
respondents they personally know. C. Eliminate supervisor field visits after
training is completed. D. Provide standardized training and closely monitor
field performance.
Answer: D. Provide
standardized training and closely monitor field performance.
Rationale: Standardized interviewer training ensures
that every interviewer administers questions consistently, while field
supervision verifies adherence to established procedures. Together, these
measures minimize interviewer bias and improve the reliability and
comparability of collected data across survey locations.
Question 17
A survey is intended to estimate
national unemployment. Which situation is MOST likely to introduce coverage
error?
A. A few respondents refuse to
answer income questions. B. The sampling frame excludes newly established
residential areas. C. Some questionnaires require additional editing after
fieldwork. D. Data entry takes longer than originally planned.
Answer: B. The sampling frame
excludes newly established residential areas.
Rationale: Coverage error occurs when the sampling
frame fails to include part of the target population or includes units outside
it. Excluding newly established residential areas means eligible households
have no chance of selection, potentially producing biased national estimates
regardless of sample size.
Question 18
Which measure is MOST appropriate
for describing the spread of observations around the mean?
A. Median. B. Mode. C. Standard
deviation. D. Interquartile range.
Answer: C. Standard deviation.
Rationale: Standard deviation measures the average
dispersion of observations around the mean and is one of the most widely used
indicators of variability in official statistics. While the interquartile range
also measures spread, standard deviation specifically quantifies variation
relative to the mean and supports many inferential statistical techniques.
Question 19
Before publishing official
statistics, an unexpected value appears substantially higher than all other
observations. What should the statistician do FIRST?
A. Remove the observation
immediately from the dataset. B. Replace the value with the sample mean. C.
Verify whether the observation is valid before deciding on treatment. D. Ignore
the value because outliers always occur naturally.
Answer: C. Verify whether the
observation is valid before deciding on treatment.
Rationale: An extreme value may represent either a
genuine observation or an error in recording, coding, or data entry.
Professional statistical practice requires verification before modifying or
deleting the value. Automatically removing, replacing, or ignoring unusual
observations may distort the final results.
Question 20
One objective of coding responses
before data entry is to:
A. Increase the number of survey
respondents. B. Convert responses into standardized formats for analysis. C.
Eliminate every possible measurement error. D. Shorten the questionnaire before
printing.
Answer: B. Convert responses
into standardized formats for analysis.
Rationale: Coding assigns standardized numerical or
categorical values to survey responses, making them suitable for efficient data
entry, storage, and statistical analysis. Coding promotes consistency across
records but does not eliminate all measurement errors or alter the
questionnaire itself.
Question 21
Two variables show a strong
positive correlation. Which conclusion is MOST appropriate?
A. One variable necessarily
causes changes in the other. B. Both variables always increase by identical
amounts. C. The variables tend to move in the same direction without proving
causation. D. The relationship remains constant for every observation.
Answer: C. The variables tend
to move in the same direction without proving causation.
Rationale: Correlation measures the strength and
direction of association between variables but does not establish a
cause-and-effect relationship. Even a very strong positive correlation may
arise because of common influencing factors or coincidence. Sound statistical
analysis distinguishes association from causation.
Question 22
A statistician must prepare a
quarterly statistical report within a strict publication schedule. Which
consideration should receive the HIGHEST priority?
A. Delay publication indefinitely
until every possible analysis is completed. B. Publish preliminary figures
without conducting quality checks. C. Release only the tables that require the
least computation. D. Balance timeliness with rigorous quality assurance
procedures.
Answer: D. Balance timeliness
with rigorous quality assurance procedures.
Rationale: Official statistics should be both timely
and reliable. Excessive delays reduce the usefulness of statistical
information, while publishing unchecked data risks disseminating inaccurate
results. Effective statistical organizations therefore balance publication
schedules with comprehensive quality assurance before release.
Question 23
Which characteristic is ESSENTIAL
for a well-designed statistical indicator?
A. It consistently measures the
intended phenomenon. B. It changes definition whenever new data become
available. C. It relies primarily on subjective interpretation. D. It excludes
internationally recognized concepts.
Answer: A. It consistently
measures the intended phenomenon.
Rationale: A useful statistical indicator should
validly and consistently represent the concept it is intended to measure.
Stable definitions improve comparability across time and locations, while
excessive subjectivity or changing definitions reduce reliability and make
interpretation difficult.
Question 24
After field data collection ends,
supervisors revisit a small sample of households to confirm that interviews
actually occurred and key responses were accurately recorded. This activity is
known primarily as:
A. Data coding. B. Data
tabulation. C. Field verification. D. Sample weighting.
Answer: C. Field verification.
Rationale: Field verification involves independently
checking completed interviews to confirm that interviewers followed survey
procedures and accurately recorded responses. It is a critical quality control
mechanism used to detect interviewer falsification, recording mistakes, or
procedural violations before final data processing.
Question 25
A ministry requests access to
individual survey responses containing respondents' names and personal
identifiers. According to the principles governing official statistics, the
statistician should:
A. Release all requested records
because they belong to the government. B. Decline disclosure unless authorized
under applicable legal and confidentiality requirements. C. Publish the dataset
online so every institution has equal access. D. Remove only the respondents'
addresses while retaining all other identifiers.
Answer: B. Decline disclosure
unless authorized under applicable legal and confidentiality requirements.
Rationale: Confidentiality is a fundamental principle of official statistics. Individual responses collected for statistical purposes are protected by law and ethical standards and may only be disclosed under specific legal authority. Protecting respondents' privacy maintains public trust, encourages honest participation in surveys, and preserves the integrity of the national statistical system.
Question 26
A statistician is tasked with
selecting 2,000 households from a national register containing 400,000
households. After randomly selecting the first household, every 200th household
is chosen thereafter. Which sampling method is being used?
A. Cluster sampling. B. Quota
sampling. C. Stratified sampling. D. Systematic
sampling.
Answer: D. Systematic
sampling.
Rationale: Systematic sampling involves selecting
units at regular intervals from an ordered sampling frame after choosing a
random starting point. In this case, every 200th household is selected
following the initial random selection. This method is efficient for large
sampling frames, provided that the ordering of units does not introduce hidden
patterns that could bias the sample.
Question 27
During data processing, a
statistician discovers that several questionnaires have missing responses for
respondents' ages. What is the BEST initial action?
A. Investigate whether the
missing values can be verified from original survey records. B. Replace every
missing value with the national average age immediately. C. Delete all
incomplete questionnaires before analysis begins. D. Assume the missing values
have no effect on statistical results.
Answer: A. Investigate whether
the missing values can be verified from original survey records.
Rationale: The first step in handling missing data
is to determine whether the information can be recovered from the original
questionnaires or field records. Verification helps preserve data quality and
avoids introducing unnecessary bias. Automatically replacing or deleting
observations without investigation may reduce the accuracy and reliability of
survey estimates.
Question 28
Which situation provides the
STRONGEST evidence that a survey question lacks validity?
A. Respondents answer the
question consistently over repeated interviews. B. Interviewers complete the
question within the planned interview time. C. The question fails to measure
the concept it was intended to measure. D. Most respondents select the same
response category.
Answer: C. The question fails
to measure the concept it was intended to measure.
Rationale: Validity refers to whether an instrument
actually measures the concept it is intended to measure. A question may produce
consistent answers (high reliability) but still lack validity if it does not
capture the intended characteristic. Ensuring validity is fundamental when
designing official statistical instruments because invalid questions produce
misleading results regardless of consistency.
Question 29
A statistician calculates the
Consumer Price Index (CPI) using prices collected from selected markets across
Tanzania. Why must the selected markets remain broadly representative over
time?
A. To ensure price changes
reflect the experience of the target population. B. To reduce the number of
commodities included in the index. C. To guarantee that inflation remains
constant each year. D. To eliminate the need for periodic basket reviews.
Answer: A. To ensure price
changes reflect the experience of the target population.
Rationale: The CPI is intended to measure changes in
the prices paid by consumers. If the selected markets are not representative,
the index may fail to reflect actual price movements experienced by households.
Representativeness therefore supports the accuracy and credibility of inflation
statistics published by official statistical agencies.
Question 30
Which characteristic BEST
distinguishes administrative data from survey data?
A. Administrative data are always
more accurate than survey data. B. Administrative data are collected primarily
for operational or legal purposes rather than statistical purposes. C.
Administrative data cannot be used in official statistics. D. Administrative
data are collected only by national statistical offices.
Answer: B. Administrative data
are collected primarily for operational or legal purposes rather than
statistical purposes.
Rationale: Administrative data originate from
government ministries, agencies, or other organizations while carrying out
their normal administrative functions, such as registration, taxation, health
services, or education management. Although not originally collected for
statistical purposes, they can provide valuable information for producing
official statistics when appropriately assessed and processed.
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