Determining Sampling Precision

Heterogeneity and frequency of the distribution of the features affect sampling precision.

How many events do I need to count?

  • 1 to 2 events per disector probe on average.
  • Starting point: 100 events in 100 probes containing 10 sections when using the optical disector (Optical Fractionator or Fractionator).
  • Over-count a pilot and draw sub-samples.
  • Use a sampling strategy that results in an acceptable variance.

How do I account for heterogeneity?

For distributions of non-homogeneous features (such as those found in biological tissues), the precision also depends on the fraction of the volume sampled. If one distribution is more heterogeneous than the other, you must sample a larger volume to achieve the same precision as for the more homogeneous distribution.

While you can't control the heterogeneity of the distribution, you can control the sampling. If you use Optical Fractionator, you can change two sampling parameters to affect the volume fraction (and therefore the precision): number of virtual sections and disector spacing (since it is most efficient to maximize the height of the disector).

How do I choose the parameters?

Once you've determined the disector spacing (i.e., area sub-fraction or asf) and number of virtual sections , conduct a pilot study.

 

See Designing a Pilot Study