instacart / wilson_score

Simple, dependency-free Wilson score

Wilson Score

Simple, dependency-free Wilson score

Inspired by How Not To Sort By Average Rating by Evan Miller

How to Use

3 positive ratings out of 5 with 95% confidence

WilsonScore.lower_bound(3, 5)

Continuity correction can improve the score, especially for a small number of samples (n < 30). As of version 0.1.0, it is enabled by default. To disable continuity correction, use:

WilsonScore.lower_bound(3, 5, correction: false)

The default confidence level is 95%. To change this, use:

WilsonScore.lower_bound(3, 5, confidence: 0.99)

To get the full interval, use:

WilsonScore.interval(3, 5)

Star Ratings

You have a rating system where users can rate products from 1 to 5 stars.

A product has two ratings - one 4 star and one 5 star.

average_rating = 4.5
total_ratings = 2
rating_range = 1..5  # 1 to 5 stars

WilsonScore.rating_lower_bound(average_rating, total_ratings, rating_range)

Installation

Add this line to your application's Gemfile:

gem 'wilson_score'

And then execute:

bundle

Additional Resources

• http://www.vassarstats.net/prop1.html
• http://www.goproblems.com/test/wilson/wilson.php

Contributing

Everyone is encouraged to help improve this project. Here are a few ways you can help:

• Report bugs
• Fix bugs and submit pull requests
• Write, clarify, or fix documentation
• Suggest or add new features

wilson_score's Issues

sqrt error with low confidence

I get Math::DomainError: Numerical argument is out of domain - "sqrt"
when running:
WilsonScore.rating_lower_bound(1, 100, 1..5, confidence: 0.12)
or
WilsonScore.rating_lower_bound(5, 25, 1..5, confidence: 0.8)

but not when running:
WilsonScore.rating_lower_bound(2, 100, 1..5, confidence: 0.12)

Lower bound method, default confidence, and use continuity correction by default

WilsonScore.lower_bound(2, 5, confidence: 0.99, correction: false)

lower_bound(0,0)

Right now, it returns an obscure error:

WilsonScore.lower_bound(0,0)
# => ArgumentError: comparison of Float with 0 failed
# from <gem>/wilson_score-0.1.0/lib/wilson_score.rb:20:in `each'

That means all apps that can possibly take 0 as the second parameter must special case it. It's not even ZeroDivisionError.

Two possible solutions:

• Return 0 as a result (not exactly correct but convenient in most cases)
• Raise a class specific error like WilsonScore::Error when 0 is given for the second parameter.

If you give me a direction, I can prepare a pull request.