DA0-002 Data Acquisition and Preparation Practice Question
An analyst needs to identify outliers in a numeric column 'transaction_amount' using the interquartile range (IQR) method. Which TWO steps are part of this process? (Select TWO).
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Subtract 1.5 times the IQR from Q1 and add 1.5 times the IQR to Q3 to define bounds
The IQR method requires first calculating the first quartile (Q1) and third quartile (Q3) of the numeric column, since the IQR is defined as Q3 minus Q1 — this makes option C correct. Once Q1, Q3, and the IQR are known, the outlier bounds are established by subtracting 1.5 × IQR from Q1 (lower fence) and adding 1.5 × IQR to Q3 (upper fence), so option A is correct. Values falling outside these fences are flagged as outliers. Option B is not required because the median is not used in the IQR fence calculation. Option D describes trimming extremes by percentile, which is a different technique and not part of the IQR method. Option E describes a z-score approach using mean and standard deviation, which is an alternative outlier-detection method, not the IQR method.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Subtract 1.5 times the IQR from Q1 and add 1.5 times the IQR to Q3 to define bounds
Why this is correct
The IQR method defines outlier bounds at Q1 minus 1.5 times the IQR and Q3 plus 1.5 times the IQR. Values falling outside these fences are flagged as outliers, directly satisfying the stem's requirement to identify the bounding step.
- ✗
Calculate the median of the column
Why it's wrong here
The IQR method needs the first and third quartiles (Q1 and Q3) to derive the bounds; the median is Q2 and plays no part in those calculations. It is tempting because median and quartiles are all positional statistics from the same family, and the median would be the right choice for a robust measure of central tendency.
- ✓
Calculate the first quartile (Q1) and third quartile (Q3)
Why this is correct
Quartiles partition the sorted transaction_amount values into four equal groups, yielding Q1 and Q3 directly. The IQR is then Q3 minus Q1, and the outlier fences sit at Q1 − 1.5×IQR and Q3 + 1.5×IQR. Without these two quartiles, the IQR method cannot define any boundary.
- ✗
Sort the data and remove the top and bottom 5%
Why it's wrong here
Trimming the top and bottom 5% is arbitrary truncation, not outlier detection; IQR computes Q1, Q3 and the 1.5×IQR fences to classify values. It is tempting because it also removes extreme values, and such percentile trimming would be correct for winsorising data before modelling, not for identifying which points are outliers.
- ✗
Compute the mean and standard deviation of the column
Why it's wrong here
Mean and standard deviation belong to the z-score method, which assumes a normal distribution; IQR uses quartiles and is distribution-agnostic. It is tempting because both approaches flag outliers numerically, and the mean/standard deviation pair would be correct for a normally distributed column where z-scores beyond a threshold are sought.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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