How to Randomly Select a Sub-Sample

Introduction

This article describes how to select a random sample in Q using JavaScript. If you need to sample within sub-groups or segments instead, see How to Randomly Select a Sub-Sample from Groups or Segments. If you're more familiar with R, please see How to Randomly Choose A Subset of Cases From a Data Set using R.

Requirements

• A Q project with a data set imported.

Method

• Create a new JavaScript Variable.
• In the dialogue that appears, select Access all data rows (advanced) and paste the code below into the Expression field.
• In the code, on the first line, change the value assigned to required_sample_size to your required sample size.
• Click OK to create the variable.
• On the Data_tab, ensure that your unique ID variable has been selected in the Case IDs drop-down in the top-left.
• On the Variables and Questions tab, right-click on the new variable and select Copy and Paste Variable(s) > As Values.... This fixes the data that has been selected by the random sampling formula.
• Hide the original JavaScript variable by selecting the yellow H in the Tags column.
• Use the new variable as a Filter by selecting the yellow F in the Tags column.

var required_sample_size = 200;

//generating an array of random numbers
var rnd = new Array(N);
var orig_rnd = new Array(N);
for (var i = 0; i < N; i++){
    r = Math.random();
    rnd[i] = r;
    orig_rnd[i] = r;
}
//Finding the cut-off
rnd.sort();
var cutoff = rnd[required_sample_size];
//creating filter variable
var result = new Array(N);
for (var i = 0; i < N; i++)
  result[i] = orig_rnd[i] < cutoff;
result

It is important to note that this code samples without replacement.

In the above code, N is a reserved variable which gives the number of cases in the data file.

Next

How to Randomly Select a Sub-Sample from Groups or Segments

How to Use JavaScript in Q

How To Set The Weighted Sample Size