Weighted random in Google Sheets - google-apps-script

Question

I have three employees: John, Ashley and Mark. During the day we keep receiving jobs to do from our clients. I want that 20% of the jobs go to John, 50% for Ashley and 30% to Mark and for that I'm trying to build a Google Sheets that randomly select according to those weights (20%, 50% and 30%). You can see it in this google sheets file.
The way I found seemed good at first, the problem is that it uses RANDBETWEEN function, and it keeps randomizing everytime anything happens in the sheet, so it might look like this:
And if I simply add one row it changes completely:
Is there a way to fix the randomization (maybe copy-pasting values through script?) or an alternative way to do the same thing?

Answer 1

You can accomplish this via a custom function created in Google Apps Script, and use Math.random() to achieve the randomness. Unlike RANDBETWEEN, this won't refresh every time the sheet is edited, or even when the spreadsheet itself is refreshed. It also wouldn't require the data in A6:B16 (the people and percentages from A2:B4 would be enough).
To accomplish this, follow these steps:
In your spreadsheet, select Tools > Script editor to open a script bound to your file.
Copy this function in the script editor, and save the project:
function GET_RANDOM_PERSON(values) { // "A2:B4"
const random = Math.random(); // Random number between 0 and 1
const sheet = SpreadsheetApp.getActiveSheet();
// Change range if needed. Range could be a function argument if preferred:
if (!values) values = sheet.getRange("A2:B4").getValues();
let acc = 0;
values = values.map(value => {
acc = acc + value[1];
return [value[0], acc]; // Return accumulated probabilities
});
// Return person getting the job:
return values.find(value => value[1] > random)[0];
}
Now, if you go back to your spreadsheet, you can use this function as if you were using a regular sheets formula, as you can see here:
Reference:
Custom Functions in Google Sheets

Answer 2

With your current formula and setup, and without scripts, you can do it with iterative calculation:
Turn on Iterative Calculation: File > Spreadsheet Settings > Calculation
Make a tickbox. (I will use cell F2)
Change your formulas to look like this:
=IF($F$2;
vlookup(RANDBETWEEN(1;10);$A$6:$B$16;2;0);
E2)
If checked, the cells will calculate new randoms on change. If unchecked, it will prevent any changes until checked again. (Checked = unlocked).
When you first enter the formula, it will initialize to 0. Check the box to generate the first list.