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The abbreviations ipm (instances per million) and ppm (parts per million) are measures of relative frequency. They express the average number of occurences of the unit or word in a hypothetical text/corpus with the size of 1 million words.

Eg. The node form běžeckých in the hundred million word corpus SYN2010 occurs 208 times, which is the equivalent of 1,72 ipm, i.e. 1,72 occurences per million words.

Use of ipm/ppm

The main advantage of relativization of frequency based on corpus size enables us to compare numbers from corpora of various sizes. In the case that corpora are not the same size, absolute values can cause confusion. The node form stromek in the corpora SYN2010 and ORAL2008 reaches these values:

SYN2010 ORAL2008
Abs. frequency 440 6
Rel. frequency (in ipm) 3,62 4,45

Regardless of what the absolute frequencies say, after considering the size of the corpora (SYN2010 has 122 mil. positions, whereas ORAL2008 has only 1,35 mil. positions), we find that the word stromek is relatively more frequent in the ORAL2008 corpus.