| 系数 | 曲线形态 | ||
| b1 | b2 | b3 | |
| >0 | =0 | =0 | 单调递增 |
| <0 | =0 | =0 | 单调递减 |
| >0 | <0 | =0 | 倒U型 |
| <0 | >0 | =0 | U型 |
| >0 | <0 | >0 | N型 |
| <0 | >0 | <0 | 倒N型 |