-/f/and /v/ sounds are labiodental fricative but different in vocal cord, voiceless and voiced but /v/ sound had percentages of incorrect more than /f/ sound.
-/s/ and /z/ sounds are apicoalveolar fricative. /s/ is voiceless but /z/ is voiced. /z/ sound had percentages of incorrect more than /s/ sound.
-/θ/ and /ð/ sounds are apicodental fricative. /θ/ is voiceless but /ð/ is voiced. /ð/ had percentages of incorrect more than /θ/ sound.