GraphPad Statistics Guide

How to: Correlation

How to: Correlation

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How to: Correlation

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Prism can perform correlation analyses either from an XY or Column table. Click the Analyze button and choose correlation.

Compute correlation between which pairs of columns?

Compute the correlation between two specific columns, between all columns (correlation matrix), or between each column and a control data set (which is X, if you are analyzing an XY table).

Assume data are sampled from a Gaussian distribuiton?

Prism offers two ways to compute correlation coefficients:

Pearson correlation calculations are based on the assumption that both X and Y values are sampled from populations that follow a Gaussian distribution, at least approximately. With large samples, this assumption is not too important.

Spearman nonparametric correlation makes no assumption about the distribution of the values, as the calculations are based on ranks, not the actual values.

One- or two-tailed P values?

Prism can compute either a one-tailed or two-tailed P value. We suggest almost always choosing a two-tailed P value. You should only choose a one-tail P value when you have specified the anticipated sign of the correlation coefficient before collecting any data and are willing to attribute any correlation in the “wrong” direction to chance, no matter how striking that correlation is.