A short quiz on vision system evaluation

Adrian F. Clark

1. Which corner of a ROC curve indicates the best performance?

   • lower right
   • upper right
   • lower left

   • upper left

   We want the smallest number of false positives for the largest number of true positives, so the best performance is the upper left corner of the plot.

2. What assumption underlies a null hypothesis test?

   • that the algorithms return a null result

   • that there is no performance difference between algorithms

   • that the algorithms don't work
   • that the algorithms differ in performance

   The null hypothesis test assumes that there is no difference in performance between algorithms, then examines whether the statistics support that assumption or provide evidence that is is wrong.

3. What is a false positive?

   • A false result from an algorithm
   • A positive result that is false

   • A true result from an algorithm that is incorrect

   • A false result from an algorithm that should be correct

   A false positive arises when an algorithm reports success but has actually found an incorrect result.

4. When using McNemar's test, what do we do if we want to see whether one algorithm's performance is better than another's?

   • Look up the Z-score in binomial tables
   • Look up the Z-score in Normal distribution tables
   • Look up the Z-score in two-tailed tables

   • Look up the Z-score in one-tailed tables

   If we want to know that one algorithm's performance is better than another, we must use one-tailed tables.

5. What is a false negative?

   • A false result from an algorithm that should have succeeded

   • A negative result that is false
   • A false result from an algorithm that should be negative
   • A false result from an algorithm

   A false negative arises when an algorithm reports failure (or crashes) when it should have succeeded.

6. When evaluating vision systems, it is normal to:

   • train on the training set and test using both training and test sets
   • train on all data but test on only the test set

   • have different training and test sets

   • use the same training and test sets

   We know that algorithms work better on the data they were trained on than on unseen data; hence, we use different training and test sets.

7. Which test is most appropriate for comparing algorithms' performances?

   • Gauss's test

   • McNemar's test

   • Laplace's test
   • Canny's test

   McNemar's test is the most appopriate test for comparing algorithms: it is a chi-squared test with one degree of freedom for paired data.

8. You are developing a automatic passport system for use by immigration, where pictures of people are compared to those in their passports. Which of the following is the best approach to take?

   • maximize the number of true positives
   • maximize the number of true positives, even if the false positive rate is high
   • minimize the number of false negatives

   • minimize the number of false positives

   For this type of application, we need to keep the number of false positives as low as possible; otherwise, we would admit lots of people who don't look like the picture on their passports.

9. What is 'ground truth'?

   • the true values obtained by an algorithm
   • values obtained by an algorithm that are known to be true
   • images of the ground

   • data known to be correct

   Ground truth are data (usually images) for which the correct answer is known; they are used for training and testing algorithms.

10. What are the axes of a ROC curve?

    • FP and FN

    • TP and FP

    • TP and TN
    • TP and FN

    An ROC curve plots the number of FPs against the number of TPs.