Learn Finnish: Sokkotestissä meillä on silmäside ja kaksi näytettä. - In the blind test, we have a blindfold and two samples.

This sentence sets up a classic scenario for kumpi. It introduces exactly kaksi näytettä (two samples). The pronoun kumpi is only used when asking "which one?" out of exactly two choices. If the blind test had three or more samples, you would have to ask "which one" using mikä.

Why is the word for "sample" (näyte) written as näytettä here?

In Finnish, numbers greater than one (like kaksi) require the noun that follows them to be in the partitive singular case. Näyte is a noun ending in -e, and these words form their partitive by adding -ttä (or -tta).

What kind of word is side in silmäside (blindfold)?

Silmäside is a compound of silmä (eye) and side (bandage or tie). Side belongs to the family of older Finnish words ending in -e. This means it gets an extra -e- and undergoes consonant gradation when you put it in other cases. For example, if you wanted to say "of the blindfold" (genitive case), the d strengthens to a t, making it silmäsiteen. Here, we just use the basic nominative form.

Can I use the word sokko from sokkotestissä to describe a blind person?