In an earlier posting we saw how ranking resumes can save a lot of time spent by recruiters and hiring managers in the recruitment process. We also saw that it lends itself well to lean hiring by enabling selection of small batch sizes.
Experiment – Manually Ranking Resumes
We had developed a game for ranking resumes by comparing pairs with some reward for the winner. The game didn’t find the level of acceptance we were expecting it to find. So we thought of getting the ranking done by a human expert. It took half a day for an experienced recruiter to rank 35 resumes. Very often the recruiter asked which attribute was to be given higher weightage? Was it experience or location or communication or compensation?
These questions indicate that every time we judge a candidate by his resume; we assign some weightage to various profile attributes like experience, expected compensation, possible start date etc. Every job opening has its own set of weightages which are implicitly assigned as we try to compare the attributes of a resume with the requirements of the job opening.
So the resume ranking problem essentially reduces to find the weightages for each one the attributes.
Challenge – Training Set for standard ranking algorithms.
There are many algorithms to solve the ranking problem. Most of the ranking algorithms fall under the class of “Supervised Learning” which would need a training set consisting of resumes graded by an expert. As we saw earlier this task is quite difficult as the grade will not only depend on the candidate profile but also on the job requirements. Moreover we can’t afford the luxury of a human expert training the algorithm for every job opening. We have to use data that is easily available without additional efforts. We do have some data of every job opening as hiring managers screen resumes and select some for interview. Its easy to extract this data from any ATS (Applicant Tracking System) . Hence we decided to use “Logistic Regression” that predicts the probability of a candidate being shortlisted based on the available data.
We have seen that “Logistic Regression” forecasts the probability based on weightages for various attributes learned from which resumes were shortlisted or rejected in the past. This probability in our case would indicate if the candidate is suitable or not. We would use this number to rank candidates in descending order of suitability.
In our company we had access to data of the following 13 attributes for about 3000 candidates that were screened for about 100 openings over the last 6 months.
1)Current Compensation, 2)Expected Compensation, 3)Education, 4)Specialization, 5)Location , 6)Earliest Start Date, 7)Total Experience, 8)Relevant Experience, 9)Communication, 10)Current Employer, 11)Stability , 12)Education Gap and 13)Work Gap.
We needed to quantify some of these attributes like education, stability , communication etc. We applied our own judgment and converted the textual data to numbers.
We were unsure whether we will get consistent results as we were falling short of historical data of resumes. We ignored openings that were barely having 10 or less resumes screened. On the other hand we also discovered a problem with large training sets – particularly in the case of job openings that drag and remain open for long. These job openings are likely to have had change of requirements. As we learned later; consistent accuracy was obtained for job openings having training sets whose population was in the range of 40 to 80 resumes.
Running Logistic Regression
We had listed 22 openings for which several hundred resumes were presented to the hiring managers in the last 6 months. We have record of interviews scheduled based on suitability of the resumes. We decided to use 75% of the available data to train (Training Set) and 25% to test (Test Set) our model. The program was written to produce the following output-
- Vector of weightages for each one of the 13 attributes
- Prediction whether the set of test cases would be “Suitable” or “Unsuitable”
The result was based on how accurate was the prediction Accuracy is defined as
Accuracy = (True Positives + True Negatives)/ (Total # of resumes in the Test Set)
Where “True Positives” is the number of suitable resumes correctly predicted to be suitable. Similarly “True Negatives” are the number of unsuitable resumes predicted as such. We achieved average accuracy of 80% ranging from 67% to 95%.
Efforts to improve accuracy
Plot of suitable and unsuitable resumes on Experience vs Pay didn’t show any consistent pattern. The suitable resumes tended to be more of highly paid individuals who had lower experience. Which is kind of counterintuitive. Other than this the suitable resumes tended to cluster closer to the center of the graph as compared to the unsuitable ones.
Given the nature of the plot the decision boundary would be non linear- probably a quadratic or higher degree polynomial. We decided to test using a 6th degree polynomial – thus creating 28 attributes from 2 main attributes – viz. experience and pay. We ran the program again this time with these 28 sixth degree polynomial and remaining 11 attributes thus a total of 39 attributes. This improved the accuracy from 80% to 88%. We achieved 100% accuracy for 4 job openings.
Regularization had no impact on accuracy. Hence we didn’t use any cross validation set for testing various values of the regularization parameter.
Values of weightages or parameters varied slightly every time we ran to find the minimum of the cost function. Which indicates that the model found a new minimum in the same vicinity every time we ran the program with no changes to the training set data.
If you take a close look at the chart above ; we observe the following-
- One job opening gives extremely negative weightage to “Current Compensation” – this means that candidates earning well are not suitable. While its just the opposite case for most other job openings.
- C++ Developer position assigns positive weightage to “Total Experience” but negative weightage to “Relevant Experience”. The requirement was for a broader skillset beyond just C++.
We can go on verifying the reasons for what turns out to be a fairly distributed set of values for weightages for various attributes. Each job opening has pretty much independent assessment of the resumes and candidates.
As expected we observed that accuracy generally increases with sample size or size of the training set.As mentioned earlier accuracy was observed to be low in the case of job openings that remained open for long and the selection criteria underwent change.