Custom randomizer frames

Experimenter supports a special kind of frame called ‘choice’ that defers determining what sequence of frames a participant will see until the page loads. This allows for dynamic ordering of frame sequence in particular to support randomization of experimental conditions or counterbalancing. The goal of this page is to walk through an example of implementing a custom ‘randomizer’.

Overview of ‘choice’ structure

Generally the structure for a ‘choice’ type frame takes the form:

    "kind": "choice",
    "sampler": "random",
    "options": [


  • sampler indicates which ‘randomizer’ to use. This must correspond with the values defined in lib/exp-player/addon/randomizers/index.js
  • options: an array of options to sample from. These should correspond with values from the frames object defined in the experiment structure (for more on this, see the experiments docs)

Making your own

There is some template code included to help you get started. From within the ember-lookit-frameplayer/lib/exp-player directory, run:

ember generate randomizer <name>

which will create a new file: addon/randomizers/<name>.js. Let’s walk through an example called ‘next. The ’next’ randomizer simply picks the next frame in a series. (based on previous times that someone participated in an experiment)

$ ember generate randomizer next
installing randomizer
  create addon/randomizers/next.js

Which looks like:

 NOTE: you will need to manually add an entry for this file in addon/randomizers/index.js, e.g.:
import Next from './next';
    next: Next
var randomizer = function(/*frame, pastSessions, resolveFrame*/) {
    // return [resolvedFrames, conditions]
export default randomizer;

The most important thing to note is that this module exports a single function. This function takes three arguments:

  • frame: the JSON entry for the ‘choice’ frame in context
  • pastSessions: an array of this participants past sessions of taking this experiment. See the experiments docs for more explanation of this data structure
  • resolveFrame: a copy of the ExperimentParser’s _resolveFrame method with the this context of the related ExperimentParser bound into the function.

Additionally, this function should return a two-item array containing:

  • a list of resolved frames
  • the conditions used to determine that resolved list

Let’s walk through the implementation:

var randomizer = function(frame, pastSessions, resolveFrame) {
    pastSessions = pastSessions.filter(function(session) {
        return session.get('conditions');
    pastSessions.sort(function(a, b) {
        return a.get('createdOn') > b.get('createdOn') ? -1: 1;
    // ...etc

First we make sure to filter the pastSessions to only the one with reported conditions, and make sure the sessions are sorted from most recent to least recent.

var option = null;
if(pastSessions.length) {
    var lastChoice = (pastSessions[0].get(`conditions.${}`) || frame.options[0]);
    var offset = frame.options.indexOf(lastChoice) + 1;
    option = frame.options.concat(frame.options).slice(offset)[0];
else {
    option = frame.options[0];

Next we look at the conditions for this frame from the last session (pastSessions[0].get(`conditions.${}`)). If that value is unspecified, we fall back to the first option in frame.options. We calculate the index of that item in the available frame.options, and increment that index by one.

This example allows the conditions to “wrap around”, such that the “next” option after the last one in the series circles back to the first. To handle this we append the options array to itself, and slice into the resulting array to grab the “next” item.

If there are not past sessions, then we just grab the first item from options.

    var [frames,] = resolveFrame(option);
    return [frames, option];

export default randomizer;

Finally, we need to resolved the selected sequence using the resolveFrame argument. This function always returns a two-item array containing: - an array of resolved frames - the conditions used to generate that array

In this case we can ignore the second part of the return value, and only care about the returned frames array.

The export default randomizer tells the module importer that this file exports a single item (export default), which in this case is the randomizer function (note: the name of this function is not important).

Finally, lets make sure to add an entry to the index.js file in the same directory:

import next from './next';

export default {
    next: next

This allows consuming code to easily import all of the randomizers at once and to index into the randomizers object dynamically, e.g. (from the ExperimentParser):

import randomizers from 'exp-player/randomizers/index';
// ...
return randomizers[randomizer](