standard-deviation-stream

standard-deviation-stream

A Node.js method for finding the standard deviation of numbers arriving in a stream. Based on Donald Knuth's solution in Art of Computer Programming as described by John D. Cook.

Basic Example

Just get the standard deviation on some numbers.

var DeviationStream = require('standard-deviation-stream');

var numbers = new DeviationStream();

numbers.push(10);
numbers.push(15);
numbers.push(20);
numbers.push(25);

console.log('Count: '+numbers.count());
console.log('Mean: '+numbers.mean());
console.log('Variance: '+numbers.variance());
console.log('Standard Deviation: '+numbers.standardDeviation());

numbers.clear();

Redis Enhanced Example

If you are fetching a stream of numbers in batches it can be useful to save the state of the standard deviation stream between batches.

For example lets say you have a web endpoint that returns results from a database collection in pages of 100 at a time, and needs to calculate standard deviation on the results. With the redis enhanced style you can fetch 100 items, calculate standard deviation on that page of 100 results, and cache the stream data.

Now the next time the web endpoint is called to fetch the next page of 100 results you can restore from cache and continue calculating the standard deviation on the next 100 items, taking into account the mean and variance of the first 100 items that you fetched in the previous request.

var async = require('async');
var DeviationStream = require('standard-deviation-stream');

var numbers = new DeviationStream('someResults', redisConnection);

async.series(
	[
		function (done) {
			numbers.restore(done);
		},

		function (done) {
			numbers.push(10);
			numbers.push(15);
			numbers.push(20);
			numbers.push(25);
			numbers.save(done);
		}
	],
	function () {
		console.log('Done');
	}
);

By default the save function stores the standard deviation stream data in a Redis key with an expiration of one hour. This allows you to continue adding more items to a stream based on that key at any time within an hour.

Installation & Testing

npm install standard-deviation-stream
npm test

To run the tests on the Redis enhanced capability you'll need to be running Redis locally on port 6379