[Chinese-translators] Please help me on translating this sentence

[mengliang99]

Hi, list friends,

I'm translating the motivation. But there is one sentence that I don't know how 
to translate. It is just as below:
"
It is only when the rewards are based on performing a given task or doing a 
good job at it - analogous to piece-rate payment and bonuses, respectively - 
that the problem develops.
"
The biggest problem is the word "piece-rate". I don't know the exact meaning 
and how to express it.

Please help me. You're appreciated.

Sincerely yours,
Liang Meng