My thoughts on using intervals for ear training.
Functional Ear Trainer app (free, not an affiliate):
PDF guide to all major scales:
Learn piano with me:
0:00 interval training
1:13 navigation metaphor
1:36 functional ear training
1:58 how to name intervals
3:32 how to use numbers instead
4:51 comparing the methods
5:58 the proof is in the pudding: example songs
7:45 ADVICE #1 - Functional Ear Trainer
8:43 ADVICE #2 - sight singing
9:15 ADVICE #3 - transpose melodies
10:10 caveat: for those with perfect pitch
11:47 final thoughts