Evaluation indicators: accuracy (Precision), Recall (Recall), and F-value (f-measure)

In order to better evaluate the performance of IR system, IR has a complete evaluation system, through which the evaluation system can understand the merits and demerits of different information systems, the characteristics of different retrieval models, the influence of different factors on information retrieval, and further optimize the information retrieval.

Since the objective of IR is to return more comprehensive and accurate information in a relatively short period of time, the evaluation indicators for information retrieval are usually taken from three aspects: efficiency, effectiveness and other data sizes.

The following is a brief introduction of several commonly used information retrieval evaluation indicators:

1. Accuracy rate and Recall rate (Precision & Recall)

Precision and recall are the two metrics widely used in the field of information retrieval and statistical classification to evaluate the quality of the results. The accuracy is the ratio of retrieving the number of related documents and the total number of documents retrieved, and the precision of the retrieval system is measured; recall is the ratio of the number of related documents retrieved and the number of related documents in the document library, which is measured by the recall of the retrieval system.

In general, precision is the number of items retrieved (such as documents, Web pages, etc.) that are accurate, and recall is how much of the exact entries have been retrieved.

The correct rate, recall rate and F value are the important evaluation indexes of the target in the mixed environment. Consider the definition of these indicators first:

1. Correct rate = The correct number of messages extracted/the number of extracted information bars

2. Recall rate = The correct number of information to be extracted/the number of information bars in the sample

The two values are between 0 and 1, the closer the value is to 1, the higher the precision or recall.

3. F value = correct rate * Recall rate * 2/(correct rate + recall rate) (f value is the harmonic mean of correct rate and recall rate)

Example: There are 1400 carp, 300 shrimp and 300 turtles in a pond. Now for the purpose of catching carp. Sprinkle a big net, caught 700 carp, 200 shrimp, 100 turtle. So, these indicators are as follows:

Correct rate = 700/(700 + 200 + 100) = 70%

Recall rate = 700/1400 = 50%

F value = 70% * 50% * 2/(70% + 50%) = 58.3%

Let's see if all the carp, shrimp and turtles in the pond are clean sweep, and how these indicators change:

Correct rate = 1400/(1400 + 300 + 300) = 70%

Recall rate = 1400/1400 = 100%

F value = 70% * 100% * 2/(70% + 100%) = 82.35%

It can be seen that the correct rate is the proportion of the target achievement in the results of the capture; recall, as the name implies, is the proportion of the target category recalled from the area of concern, while the F-value is the evaluation index of the two indicators, which is used to comprehensively reflect the overall indicators.

Of course, it is hoped that the higher the result precision the better, and the higher the recall, the better, but in fact the two are contradictory in some cases. For example, in extreme cases, we only search for a result and are accurate, then precision is 100%, but recall is very low, and if we return all the results, such as recall is 100%, but precision will be very low. Therefore, in different occasions need to judge their own hope precision higher or recall higher. If you are doing an experimental study, you can draw Precision-recall curves to help with the analysis.

2. Comprehensive Evaluation Index (F-MEASURE)

P and r indicators sometimes appear contradictory situations, which require a comprehensive consideration of them, the most common way is f-measure (also known as F-score).

F-measure is a weighted harmonic average of precision and recall :

When the parameter is α=1, it is the most common F1, which is

It is shown that the results of P and R are synthesized by F1, and when the F1 is higher, the test method is more effective.

3, E value

The E value represents the weighted average of the precision ratio p and recall r, when one is 0 o'clock, the e value is 1, and the formula is calculated as follows:

The larger the B, the greater the weight of the precision.

4. Average correct rate (Average Precision, AP)

The average correct rate represents the average of the correct rate on different recall points. For

Evaluation indicators: accuracy (Precision), Recall (Recall), and F-value (f-measure)