The proportion of items (or batches, in Skip lot sampling) that are inspected during some phase, when applying continuous sampling. f is between 0 and 1. There are three ways to sample with a fraction of f:

1. Probability Sampling: Each item/batch is sampled with probability f.
2. Systematic Sampling: Every 1/f 'th item/batch is sampled. 1/f must then be a natural number (e.g., every 3rd item is inspected, when f=1/3).
3. Block-Random Sampling: From each 1/f consecutive items/batches, one is chosen at random. 1/f must then be a natural number (e.g., in each block of 3 items one is chosen, whenf=1/3).