jsPsych.randomization

The jsPsych.randomization module contains methods that are useful for generating random lists of trial variables.


jsPsych.randomization.factorial

jsPsych.randomization.factorial(factors, repetitions, unpack)

Parameters

Parameter Type Description
factors object The factors object should contain a property for each different factor. Each property-factor should have a value of an array, with each element of the array corresponding to a level of the factor.
repetitions integer The number of times to repeat each unique combination of the factors in the output sample.
unpack boolean If true then the output will be an object with a property for each factor in the original factors object. The value of each property-factor will be an array containing the levels of the factor in a random order. The order will be consistent across each property-factor (e.g., the first element of each property-factor will specify one unique combination of the factors). If false, then the return value will be an array of objects where each property-factor contains only a single value.

Return value

The return value depends on the unpack parameter. See description of the parameter above, and examples below.

Description

This method takes a list of factors and their levels, and creates a full factorial design by creating each unique combination of the factors. The returned set of combinations is in a random order.

Examples

Create full factorial design

var factors = {
    stimulus: ['a.jpg', 'b.jpg'],
    ms_delay: [100, 200]
}

var full_design = jsPsych.randomization.factorial(factors, 1);

/*
output:
full_design = [
    {stimulus: 'a.jpg', ms_delay: 200},
    {stimulus: 'b.jpg', ms_delay: 200},
    {stimulus: 'b.jpg', ms_delay: 100},
    {stimulus: 'a.jpg', ms_delay: 100},
]
*/

Create full factorial design with repeats

var factors = {
    stimulus: ['a.jpg', 'b.jpg'],
    ms_delay: [100, 200]
}

var full_design = jsPsych.randomization.factorial(factors, 2);

/*
output:
full_design = [
    {stimulus: 'b.jpg', ms_delay: 200},
    {stimulus: 'b.jpg', ms_delay: 100},
    {stimulus: 'b.jpg', ms_delay: 100},
    {stimulus: 'a.jpg', ms_delay: 100},
    {stimulus: 'a.jpg', ms_delay: 200},
    {stimulus: 'b.jpg', ms_delay: 200},
    {stimulus: 'a.jpg', ms_delay: 100},
    {stimulus: 'a.jpg', ms_delay: 200},
]
*/

Create full factorial design, unpacked

var factors = {
    stimulus: ['a.jpg', 'b.jpg'],
    ms_delay: [100, 200]
}

var full_design = jsPsych.randomization.factorial(factors, 1, true);

/*
output:
full_design = {
    stimulus: ['a.jpg','b.jpg','b.jpg','a.jpg'],
    ms_delay: [200, 100, 200, 100]
]
*/

jsPsych.randomization.randomID

jsPsych.randomization.randomID(length)

Parameters

Parameter Type Description
length integer The length of the randomly generated ID

Return value

Returns a string of length length where each character is randomly selected from the numbers 0-9 and all lowercase English letters a-z.

Description

Generates a random string that is likely to be unique. If length is undefined, then the string length is 32.

Example

console.log(jsPsych.randomization.randomID());
// outputs: "t7dwz0e713pc8juuaayyfvpkdd9un239"

console.log(jsPsych.randomization.randomID(8));
// outputs: "3xtpcbck"

jsPsych.randomization.repeat

jsPsych.randomization.repeat(array, repetitions, unpack)

Parameters

Parameter Type Description
array array The array of values to randomize & repeat.
repetitions integer or array The number of times to repeat each element of the array in the final sample. If this parameter is defined as an integer, then each element of array is repeated the same number of times. This parameter can also be an array of the same length as array, in which case each element of array will be repeated the number of times defined in the corresponding position of the repetitions array.
unpack boolean If each element of array is an object with an equivalent set of properties, then setting unpack to true will make the return value an object with a property for each of the unique properties among the elements of the array. Each property in the output object will be an array containing the values for that property in the randomized order. The order will be consistent across properties. If this is false then the output is just an array containing a randomized order of the original array elements.

Return value

The return value depends on the unpack parameter. See description of the parameter above, and examples below.

Description

This method takes an array of values and generates a new random order of the array, with the option of repeating each element of the array a specified number of times.

If the array elements are objects with the same set of properties, then this method can optionally return a single object where each property is a randomized order of the properties defined in the original set of objects. This is useful for randomizing sets of parameters that are used to define a jsPsych block.

Examples

Shuffle an array, no repeats

var myArray = [1,2,3,4,5];
var shuffledArray = jsPsych.randomization.repeat(myArray, 1);

// output: shuffledArray = [3,2,4,1,5]

Shuffle an array with repeats

var myArray = [1,2,3,4,5];
var shuffledArray = jsPsych.randomization.repeat(myArray, 2);

// output: shuffledArray = [1,3,4,2,2,4,5,1,5,3]

Shuffle an array of objects

var trial1 = {
    stimulus: 'img/faceA.jpg',
    correct_key: 80,
    person_name: 'Joe'
}

var trial2 = {
    stimulus: 'img/faceB.jpg',
    correct_key: 80,
    person_name: 'Fred'
}

var trial3 = {
    stimulus: 'img/faceC.jpg',
    correct_key: 81,
    person_name: 'Mary'
}

var myArray = [ trial1, trial2, trial3 ];
var shuffledArray = jsPsych.randomization.repeat(myArray, 2);

// output: shuffledArray = [ trial1, trial3, trial3, trial2, trial1, trial2 ]

Shuffle an array of objects, with unpack

var trial1 = {
    stimulus: 'img/faceA.jpg',
    correct_key: 80,
    person_name: 'Joe'
}

var trial2 = {
    stimulus: 'img/faceB.jpg',
    correct_key: 80,
    person_name: 'Fred'
}

var trial3 = {
    stimulus: 'img/faceC.jpg',
    correct_key: 81,
    person_name: 'Mary'
}

var myArray = [ trial1, trial2, trial3 ];
var shuffledArray = jsPsych.randomization.repeat(myArray, 2, true);

/*
output: shuffledArray = {
    stimulus: ['img/faceB.jpg','img/faceA.jpg','img/faceC.jpg','img/faceA.jpg','img/faceC.jpg','img/faceB.jpg'],
    correct_key: [80, 80, 81, 80, 81, 80],
    person_name: ['Fred','Joe', 'Mary', 'Joe', 'Mary', 'Fred']
}
*/

jsPsych.randomization.sampleWithReplacement

jsPsych.randomization.sampleWithReplacement(array, sampleSize, weights)

Parameters

Parameter Type Description
array array The array of values to sample from
sampleSize numeric The number of samples to draw
weights array The relative weight of each element in array. This array is normalized, so the values do not need to sum to 1. The length must match the length of array.

Return value

An array containing the sample.

Description

This method returns a sample drawn at random from a set of values with replacement. The relative probability of drawing each item can be controlled by specifying the weights.

Examples

Sample with equal probability

var myArray = [1,2,3,4,5];
var sample = jsPsych.randomization.sampleWithReplacement(myArray, 10);

// output: sample = [3, 1, 2, 2, 5, 1, 4, 3, 1, 5];

Sample with unequal probability

var myArray = [1,2,3,4,5];
var sample = jsPsych.randomization.sampleWithReplacement(myArray, 10, [6,1,1,1,1]);

// output: sample = [3, 4, 5, 1, 2, 1, 3, 1, 1, 1];

jsPsych.randomization.sampleWithoutReplacement

jsPsych.randomization.sampleWithoutReplacement(array, sampleSize)

Parameters

Parameter Type Description
array array The array of values to sample from
sampleSize numeric The number of samples to draw

Return value

An array containing the sample.

Description

This method returns a sample drawn at random from a set of values without replacement. The sample size must be less than or equal to the length of the array.

Examples

Sample without replacement

var myArray = [1,2,3,4,5];
var sample = jsPsych.randomization.sampleWithoutReplacement(myArray, 2);

// output: sample = [3,2];

jsPsych.randomization.shuffle

jsPsych.randomization.shuffle(array)

Parameters

Parameter Type Description
array array The array of values to shuffle

Return value

Returns an array with the same elements as the input array in a random order.

Description

A simple method for shuffling the order of an array.

Examples

Shuffle an array

var myArray = [1,2,3,4,5];
var shuffledArray = jsPsych.randomization.shuffle(myArray);

// output: shuffledArray = [3,2,4,1,5]

jsPsych.randomization.shuffleNoRepeats

jsPsych.randomization.shuffleNoRepeats(array, equalityTest)

Parameters

Parameter Type Description
array array The array of values to shuffle
equalityTest function A function to use to evaluate the equality of neighbors in the array. The function should accept two parameters, which are the two elements to be tested. It should return true if they are equal and false if not. The default function, if none is specified, is to use the === operator. This will work for primitive values, but fail for Objects and Arrays. An example function is given below in the examples.

Return value

Returns an array with the same elements as the input array in a random order, with no repeating neighbors.

Description

Shuffle an array, ensuring that neighboring elements in the array are different.

Warning: if you provide an array that has very few valid permutations with no neighboring elements, then this method will fail and cause the browser to hang.

Examples

Basic example

var myArray = [1,2,3,4,5,1,2,3,4,5,1,2,3,4,5];
var shuffledArray = jsPsych.randomization.shuffleNoRepeats(myArray);

// output: shuffledArray = [2, 3, 5, 1, 2, 4, 1, 5, 4, 1, 3, 5, 4, 3, 2]

Custom equalityTest

var myObjects = [
  {color:"blue"},
    {color:"red"},
    {color:"yellow"},
    {color:"orange"}
];

var repeatedSet = jsPsych.randomization.repeat(myObjects,3);
var shuffled = jsPsych.randomization.shuffleNoRepeats(repeatedSet, function(a,b) { return a.color === b.color });

// console.log(JSON.stringify(shuffled))
// "[{"color":"red"},{"color":"yellow"},{"color":"blue"},{"color":"yellow"},{"color":"orange"},{"color":"red"},{"color":"yellow"},{"color":"orange"},{"color":"blue"},{"color":"orange"},{"color":"red"},{"color":"blue"}]"