A client asked: “why there is one occurrence of “sta” and one occurrence of “stä” in the phrases below? Are both correct?

• voi esiintyä useammalla kuin 1 potilaalla 10:stä
• voi esiintyä enintään 1 potilaalla 100:sta

My quick answer (without long grammatical explanations):

-stä and -sta are both correct in their respective places. The reason:  the vowel used in the ending depends on the vovels present in the beginning of the word. This phenomenon  is called “vowel harmony“.

100 = sata -> sadasta
10 = kymmenen -> kymmenestä
1 = yksi -> yhdestä
1000 = tuhat -> tuhannesta

A certain vowel combination requires -ä, another vowel combination requires -a.

There are some foreign words where both endings can be used, e.g. analyysi; ‘analyysistä’ and ‘analyysista’ are both ok. But this is rare.