I have attempted to exercise a non-linear model fit to my data, `dadtAbs`

, and got the following puzzle

`nlm = NonlinearModelFit[ Transpose[{Table[t, {t, 1, tmax}],dadtAbs}], (1 - aaa)*bbb*NSPI[bbb, ddd, population, t] * (1 - ddd)*(population - NSPI[bbb, ddd, population, t]), {aaa, bbb, ddd}, t] `

where `dadtAbs`

is a list, population is a known constant, `aaa`

, `bbb`

, `ddd`

are the desired answers, and `t`

is the variable.

When I queried `nlm`

, I got

`[0.]`

Here is the function `NSPI`

:

`NSPI[alpha_, delta_, population_, tt_] := population/(1 + (population - 1)*Exp[-alpha*population*(1 - delta)*tt]) `

population is a large constant, for example, 1000000