Repeat Subset of Trials

[SHogenboom]

I'm sure I'm just missing something but I'm trying to:

1. Select a sample of trials (e.g., 3 / 108)
2. Repeat this sample in a random order (e.g., N = 2)

I have tried two logics. First combine all requirements into one timeline:

var timeline_experiment = {
  timeline: [trial], // show the trials
  timeline_variables: target_stimuli, // from file
  data: { data_type: "experiment" },
  // SAMPLE FROM STIMULI
  sample: {
    type: "without-replacement",
    size: 2, // 2 stimuli
  },
  // REPEAT TIMELINE
  // repetitions in random order
  randomize_order: true,
  repetitions: 3,
};

This gave me 6 trials with 6 different stimuli, which I guess is a result from 2 stimuli repeated 3 times. However, I want to have the same 2 stimuli repeated 3 times.

Attempt 2: a two-step timeline idea where I sample first, and then try to repeat the sampled timeline.

// TIMELINE: experiment trials
var timeline_experiment_sample = {
  timeline: [trial], // show the trials
  timeline_variables: target_stimuli,
  data: { data_type: "experiment" },
  // SAMPLE FROM STIMULI
  sample: {
    type: "without-replacement",
    size: 2, // 10 trials
  },
}; // END all_trials

var timeline_experiment_repetitions = {
  timeline: [timeline_experiment_sample],
  // REPEAT TIMELINE
  // repetitions in random order
  randomize_order: true,
  repetitions: 3,
};

Pushing only the last timeline (timeline_experiment_repetitions) yields exactly the same result as above.

What am I missing?

[SHogenboom]

After watching the tutorial: https://www.youtube.com/watch?v=LP7o0iAALik I understood the role of loops somewhat better.

SOLUTION

// TIMELINE: EXPERIMENT
var loop_counter = 1; // set global variable

var timeline_experiment = {
  // show the canvas-keyboard trials as defined above
  timeline: [trial],
  // Select a random set of stimulus words per participant
  // NOTE: sourced from `stimuli.js`
  // NOTE: specify function below so that is called only once each experiment.
  timeline_variables: sampleStimuli((n_unique = 3)),
  // Present the trials in a random order
  randomize_order: true,
  // NOTE: A loop is always executed once, so reduce desired number of repetitions by 1
  loop_function: function () {
    // Desired number of repetitions
    if (loop_counter < 3) {
      loop_counter = loop_counter + 1; // increase loop counter to prevent infinite loop
      return true; // continues with the next loop
    } else {
      return false; // discontinues the loop
    }
  },
  // save for easy data processing
  data: { data_type: "experiment" },
};

// Custom function for sampling the required stimuli
function sampleStimuli(n_unique) {
  var selected_stimuli = jsPsych.randomization.sampleWithoutReplacement(
    target_stimuli,
    n_unique
  );
  return selected_stimuli;
}