If you are coaching or involved with a chorus that tends to go flat, here are some things you can look for in the composition of the song.
Most choruses sing more in tune when they are singing major keys, rather than in minor ones.
This infers that accustomed hearing affects pitch since we are used to hearing and singing in major keys. Therefore, it is a good idea to have a chorus vocalize in minor keys as well as in the major ones. If the chorus has pitch difficulties, avoid pieces filled with chromatic progressions. Stick to the diatonic scale. If the vowels in the text are predominantly EH, IH, or EE, this may contribute to flatting since many choruses and quartets control these vowels poorly.