CWEA Laboratory Analyst Grade 3 Practice Exam with 100
Questions and Answers | Updated 2026 A+ | Instant Download
PDF
Table of Contents
•
1. Quality Assurance, Quality Control, and Data Integrity
•
2. Advanced Instrumental Analysis and Troubleshooting
•
3. Wastewater Microbiology, Bioassays, and Pathogen Testing
•
4. Laboratory Safety, Regulatory Compliance, and Facility Management
1. A laboratory analyst performs a matrix spike (MS) recovery calculation for a
wastewater sample and gets 145%, which exceeds the acceptance limit of 75-125%,
while the laboratory control sample (LCS) recovery is 98%. Which action is most
appropriate?
A. Re-analyze the sample without qualification since LCS passed.
B. Flag the sample result for potential positive matrix interference and report with a
qualifier.
C. Qualify all analytical batches processed on that day as invalid.
D. Recalibrate the instrument and re-run the entire sequence immediately.
Answer: B
,Rationale: An acceptable LCS confirms analytical method performance, indicating that high
MS recovery is caused by sample-specific matrix interference requiring qualification rather
than full batch invalidation.
2. During a BOD5 analysis, the unseeded dilution water blank shows a dissolved oxygen
depletion of 0.35 mg/L after 5 days at 20°C. What is the correct quality control
response?
A. Accept the data because 0.35 mg/L is below the action threshold of 0.50 mg/L.
B. Investigate the source of organic contamination in the dilution water, as blank depletion
must not exceed 0.20 mg/L.
C. Subtract 0.35 mg/L from all sample BOD results to correct for background depletion.
D. Increase the incubator temperature to accelerate residual organic oxidation.
Answer: B
Rationale: Standard Methods mandates that unseeded dilution water depletion must not
exceed 0.20 mg/L, making 0.35 mg/L an indicator of contamination that requires
investigation and corrective action.
3. An analyst runs an ICP-OES calibration curve using five standards and obtains an R²
value of 0.988, but the lowest standard shows a 35% relative error. What is the
required corrective action under strict QA/QC protocols?
A. Report the data since R² is greater than 0.980.
B. Re-evaluate the low standard, perform a non-linear fit or re-calibrate to ensure low-
level accuracy within acceptable error limits.
C. Omit the highest calibration standard to artificially improve the R² value.
D. Adjust the initial calibration verification (ICV) concentration to match the error.
Answer: B
Rationale: High correlation coefficients can mask significant low-end non-linearity, so
calibration standards must meet individual accuracy criteria before sample quantification.
4. What is the primary purpose of analyzing a Method Detection Limit (MDL) duplicate
set annually or following major instrument maintenance?
A. To verify maximum instrument signal intensity.
,B. To demonstrate low-level precision and recalculate realistic operational detection
thresholds.
C. To replace daily calibration verification requirements.
D. To validate matrix spike recovery percentages at mid-range concentrations.
Answer: B
Rationale: Annual or post-maintenance MDL studies establish lower limits of quantification
by measuring low-level measurement variability across multiple replicates.
5. When assessing control charts using Shewhart rules, four consecutive matrix spike
recoveries fall between +1 and +2 standard deviations on the same side of the mean.
How should the QC manager interpret this?
A. As a normal statistical variation requiring no documentation.
B. As a systematic trend or bias warning that warrants process monitoring prior to out-of-
control conditions.
C. As an immediate catastrophic failure requiring batch rejection.
D. As proof that the system's variance is decreasing.
Answer: B
Rationale: Consecutive points falling on one side of the mean signal a systematic shift or
emerging bias in the measurement process before hard control limits are breached.
6. An analyst calculates the Relative Percent Difference (RPD) between a sample and
sample duplicate to be 28% for Total Suspended Solids (TSS), where the laboratory
acceptance limit is ≤ 20%. What is the proper procedure?
A. Average the two values and report without qualification.
B. Re-analyze the original sample and duplicate if sample volume permits, or qualify the
data due to sample non-homogeneity.
C. Discard the lower value and report the higher TSS value.
D. Change the control limit in the Quality Manual to 30%.
Answer: B
Rationale: Exceeding RPD thresholds requires re-analysis or qualification to transparently
document precision failures caused by sampling error or matrix heterogeneity.
, 7. In Method 1664A for Oil and Grease, a recovery of 62% is obtained for the Ongoing
Precision and Recovery (OPR) standard. What must the analyst do?
A. Invalidate the analytical batch and re-extract all associated samples following corrective
action.
B. Report the data with an estimator flag for high bias.
C. Adjust sample values mathematically using the 62% recovery factor.
D. Proceed with sample analysis if the laboratory blank is clean.
Answer: A
Rationale: An OPR failure indicates a system failure across the entire extraction batch,
requiring corrective action and complete re-extraction.
8. Which statistical parameter directly determines whether two analytical methods
yield statistically equivalent results at a 95% confidence level?
A. Standard error of the mean.
B. Two-tailed paired t-test.
C. Relative Standard Deviation (RSD).
D. Linear regression slope coefficient.
Answer: B
Rationale: A paired t-test compares mean differences between two method datasets against
critical t-values to evaluate statistical equivalence.
9. A initial demonstration of capability (IDC) for a new analyst requires analyzing how
many replicate spikes, and what criterion must be met?
A. 2 replicates; recovery within 50–150%.
B. 4 replicates; precision and accuracy meeting method-defined acceptance criteria.
C. 10 replicates; zero standard deviation.
D. 3 replicates; RPD less than 5%.
Answer: B
Questions and Answers | Updated 2026 A+ | Instant Download
Table of Contents
•
1. Quality Assurance, Quality Control, and Data Integrity
•
2. Advanced Instrumental Analysis and Troubleshooting
•
3. Wastewater Microbiology, Bioassays, and Pathogen Testing
•
4. Laboratory Safety, Regulatory Compliance, and Facility Management
1. A laboratory analyst performs a matrix spike (MS) recovery calculation for a
wastewater sample and gets 145%, which exceeds the acceptance limit of 75-125%,
while the laboratory control sample (LCS) recovery is 98%. Which action is most
appropriate?
A. Re-analyze the sample without qualification since LCS passed.
B. Flag the sample result for potential positive matrix interference and report with a
qualifier.
C. Qualify all analytical batches processed on that day as invalid.
D. Recalibrate the instrument and re-run the entire sequence immediately.
Answer: B
,Rationale: An acceptable LCS confirms analytical method performance, indicating that high
MS recovery is caused by sample-specific matrix interference requiring qualification rather
than full batch invalidation.
2. During a BOD5 analysis, the unseeded dilution water blank shows a dissolved oxygen
depletion of 0.35 mg/L after 5 days at 20°C. What is the correct quality control
response?
A. Accept the data because 0.35 mg/L is below the action threshold of 0.50 mg/L.
B. Investigate the source of organic contamination in the dilution water, as blank depletion
must not exceed 0.20 mg/L.
C. Subtract 0.35 mg/L from all sample BOD results to correct for background depletion.
D. Increase the incubator temperature to accelerate residual organic oxidation.
Answer: B
Rationale: Standard Methods mandates that unseeded dilution water depletion must not
exceed 0.20 mg/L, making 0.35 mg/L an indicator of contamination that requires
investigation and corrective action.
3. An analyst runs an ICP-OES calibration curve using five standards and obtains an R²
value of 0.988, but the lowest standard shows a 35% relative error. What is the
required corrective action under strict QA/QC protocols?
A. Report the data since R² is greater than 0.980.
B. Re-evaluate the low standard, perform a non-linear fit or re-calibrate to ensure low-
level accuracy within acceptable error limits.
C. Omit the highest calibration standard to artificially improve the R² value.
D. Adjust the initial calibration verification (ICV) concentration to match the error.
Answer: B
Rationale: High correlation coefficients can mask significant low-end non-linearity, so
calibration standards must meet individual accuracy criteria before sample quantification.
4. What is the primary purpose of analyzing a Method Detection Limit (MDL) duplicate
set annually or following major instrument maintenance?
A. To verify maximum instrument signal intensity.
,B. To demonstrate low-level precision and recalculate realistic operational detection
thresholds.
C. To replace daily calibration verification requirements.
D. To validate matrix spike recovery percentages at mid-range concentrations.
Answer: B
Rationale: Annual or post-maintenance MDL studies establish lower limits of quantification
by measuring low-level measurement variability across multiple replicates.
5. When assessing control charts using Shewhart rules, four consecutive matrix spike
recoveries fall between +1 and +2 standard deviations on the same side of the mean.
How should the QC manager interpret this?
A. As a normal statistical variation requiring no documentation.
B. As a systematic trend or bias warning that warrants process monitoring prior to out-of-
control conditions.
C. As an immediate catastrophic failure requiring batch rejection.
D. As proof that the system's variance is decreasing.
Answer: B
Rationale: Consecutive points falling on one side of the mean signal a systematic shift or
emerging bias in the measurement process before hard control limits are breached.
6. An analyst calculates the Relative Percent Difference (RPD) between a sample and
sample duplicate to be 28% for Total Suspended Solids (TSS), where the laboratory
acceptance limit is ≤ 20%. What is the proper procedure?
A. Average the two values and report without qualification.
B. Re-analyze the original sample and duplicate if sample volume permits, or qualify the
data due to sample non-homogeneity.
C. Discard the lower value and report the higher TSS value.
D. Change the control limit in the Quality Manual to 30%.
Answer: B
Rationale: Exceeding RPD thresholds requires re-analysis or qualification to transparently
document precision failures caused by sampling error or matrix heterogeneity.
, 7. In Method 1664A for Oil and Grease, a recovery of 62% is obtained for the Ongoing
Precision and Recovery (OPR) standard. What must the analyst do?
A. Invalidate the analytical batch and re-extract all associated samples following corrective
action.
B. Report the data with an estimator flag for high bias.
C. Adjust sample values mathematically using the 62% recovery factor.
D. Proceed with sample analysis if the laboratory blank is clean.
Answer: A
Rationale: An OPR failure indicates a system failure across the entire extraction batch,
requiring corrective action and complete re-extraction.
8. Which statistical parameter directly determines whether two analytical methods
yield statistically equivalent results at a 95% confidence level?
A. Standard error of the mean.
B. Two-tailed paired t-test.
C. Relative Standard Deviation (RSD).
D. Linear regression slope coefficient.
Answer: B
Rationale: A paired t-test compares mean differences between two method datasets against
critical t-values to evaluate statistical equivalence.
9. A initial demonstration of capability (IDC) for a new analyst requires analyzing how
many replicate spikes, and what criterion must be met?
A. 2 replicates; recovery within 50–150%.
B. 4 replicates; precision and accuracy meeting method-defined acceptance criteria.
C. 10 replicates; zero standard deviation.
D. 3 replicates; RPD less than 5%.
Answer: B
