Rare Events Calculation Tool

"How many events do you need to acquire?"

Assay variation can be a major source of error, therefore it is critical to understand the statistics behind rare event analysis. The cytometry friendly Gaussian distribution cannot be applied to the analysis of such a low number of events (the term "rare" is given to events with a frequency of 0.01% or less) and the best approach is to use Poisson statistics (Fig.1). Good experimental practice suggests to keep Coefficient of Variation (CV) below 5%, and thus the number of events to acquire should be defined in order to maintain the lowest CV possible and in any case below 5%.

The number of events that satisfy a given criterion (e.g they are positive for a marker, P) is defined as

$P=R/N$

where

$N$=total events
$R$=events that meet a given criterion

and

$0<=P<=1$

As with all statistical distribution,
the variance (V) is defined as

$V(R) = NP(1-P)$

The standard deviation (SD) is

$SD=√V=√(NP(1-P))$

and the coefficient of variation is

$CV=1/√V$

Figure 1. Poisson statistics defines the probability that a number of events will occur in a fixed interval of time/space/volume.

Use the tool below to help define the ideal number of events to acquire for the expected frequency of rare population.

Step 1: Input the expected frequency in the cell below; i.e. to identify a cell population that represents 0.01% of the total, input frequency is 0.001 (one positive event every ten thousands)

Frequency*