Derived Collections

Fathom Overview > Derived Collections

Some of Fathom’s collections automatically fill with data, according to rules you specify. These are called derived collections, and there are several kinds. The two most important are sample and measures collections. A sample collection is just what it sounds like: a collection that is a sample of some other collection, called its source. Select the source and choose Sample Cases from the Collection menu. The sample collection appears. Control how many cases it samples and whether it samples with or without replacement in its inspector.

The measures collection is the key to simulation and analysis. It converts measures into case attributes, so you can record statistics (measures) about your collections. (It’s a bit tricky and is best understood after you’ve tried it.) Suppose you have a collection with five (randomly generated) dice in it and a measure, or total, that contains their sum. If you collect measures from that source collection, the measures collection can record the total as a case. If you tell the measures collection to collect 100 measures, it will instruct the dice to reroll 100 times, it will compute each total, and it will record those 100 values. See Create Simulations.

In addition to sample and measures collections, several other derived collections come in handy. A scrambled collection reorders the values of one of the source collection’s attributes, allowing simulations of situations in which the values of two attributes are guaranteed to be independent. A cells collection is generated from a summary table: It has one case for each cell in the table and one attribute for each formula. A results collection is like a measures collection, except that it treats a statistical object as the source of its measures.