Need help with NSSortDescriptor-WilsonRank?
Click the “chat” button below for chat support from the developer who created it, or find similar developers for support.

mattt
440 Stars 12 Forks MIT License 11 Commits 0 Opened issues

#### Description

The Correct Way To Rank Up/Down Voted Items

!
?

# 327
Objecti...
Swift
Xcode
codable
9 commits
# 136
Swift
Objecti...
swift-f...
iphone
1 commit

# NSSortDescriptor+WilsonRank

The Correct Way To Rank Up/Down Voted Items

Let's say you have a collection of items, each of which have been rated thumbs positively or negatively a certain number of times. Maybe it's products on an e-commerce store, or links submitted to a popular hacker community.

How do we rank them?

Sorting by positive vs. negative % alone gives undue advantage to items with fewer ratings, whereas sorting by total number of positive ratings makes it difficult for new items to break into the top.

Fortunately, there is a correct solution: use the lower bound of Wilson score confidence interval for a Bernoulli parameter.

As eloquently described by Evan Miller:

We need to balance the proportion of positive ratings with the uncertainty of a small number of observations. Fortunately, the math for this was worked out in 1927 by Edwin B. Wilson. What we want to ask is: Given the ratings I have, there is a 95% chance that the "real" fraction of positive ratings is at least what? Wilson gives the answer. Considering only positive and negative ratings (i.e. not a 5-star scale), the lower bound on the proportion of positive ratings is given by:

Here p̂ is the observed fraction of positive ratings, zα/2 is the (1-α/2) quantile of the standard normal distribution, and n is the total number of ratings.

## Usage

NSArray *fruits = @[@{@"name": @"apple", @"up": @(77), @"down": @(14)},
@{@"name": @"banana", @"up": @(90), @"down": @(78)},
@{@"name": @"cherry", @"up": @(28), @"down": @(6)},
@{@"name": @"durian", @"up": @(2), @"down": @(43)},
@{@"name": @"elderberry", @"up": @(81), @"down": @(42)},
@{@"name": @"fig", @"up": @(70), @"down": @(93)},
@{@"name": @"grape", @"up": @(48), @"down": @(89)},
@{@"name": @"honeydew", @"up": @(65), @"down": @(26)},
];

NSSortDescriptor *sortDescriptor =
[NSSortDescriptor wilsonRankSortDescriptorWithPositiveKey:@"up"
negativeKey:@"down"
ascending:NO];
for (id fruit in [fruits sortedArrayUsingDescriptors:@[sortDescriptor]]) {
NSLog(@"%@ (%@↑ / %@↓)", fruit[@"name"], fruit[@"up"], fruit[@"down"]);
}


### Results

• apple (77↑ / 14↓)
• cherry (28↑ / 6↓)
• honeydew (65↑ / 26↓)
• elderberry (81↑ / 42↓)
• banana (90↑ / 78↓)
• fig (70↑ / 93↓)
• grape (48↑ / 89↓)
• durian (2↑ / 43↓)

Mattt