Arithmetic error when calculating inverse of the logistic?

I would like to rearrange the logistic function:

$$y=\frac1{1+\exp(-a+bx)}$$

To calculate $x=f(y)$

So I did the following:

$$\frac1{y}=1+\exp(-a+bx)$$ $$\ln\left(\frac1{y}-1\right)=-a+bx$$ $$\frac{\ln\left(\frac1{y}-1\right) +a}{b}=x$$

but when I try to plot this function, it appears that I have lost a '-' somewhere, i.e., I have calculated $-x=f(y)$

Where I have missed the negative sign?

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Are you sure your original function is written correctly? Shouldn't $x$ have a negative coefficient? –  Qiaochu Yuan Apr 19 '11 at 22:41
@Qiaochu Do you mean $y=\frac{-1}{1+\text{exp}(-a+bx)}$? –  Abe Apr 20 '11 at 15:29

I am not allowed to comment, so I will answer. First, your algebra is correct. If you plotted it, you must have specified numeric values for a, b. I tried a=2, b=1 and the plots looked totally inverse, just as they should [except the sigmoid was decreasing i.e. reflected in y-axis as compared to usual logistic], so if your plots did not look inverse to each other, what values of a,b did you use? If you tell us that, perhaps we could say more.

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@Rita; +1 and hope that you give answers instead of comments in the future so you can eventually down vote. Also, you get +100 in reputation by adding another site. So click here while logged in as Rita the dog: meta.stackoverflow.com . –  Carl Brannen Apr 20 '11 at 0:17
@Carl: thanks, signed onto meta but so far no bonus. Maybe there is a delay. –  Rita the dog Apr 20 '11 at 0:57
@Rita; Ooops. Now that I read more, it seems that this only applies in getting starting rep in a new area when you already have rep somewhere else in excess of 200. See: meta.stackexchange.com/questions/35033/… But having looked at your other comments, I believe you will be at the 200 level in a week or two at most. –  Carl Brannen Apr 20 '11 at 2:20
@Rita I was using $[a,b]= [-20.6, 0.25]$, a maximum likelihood fit for a logistic regression model to data. I was trying to invert the equation to predict x from y, but apparently I made the mistake of starting with the wrong equation. –  Abe Apr 20 '11 at 15:36
Your answer is indeed correct. For $y \in (0,1)$ you'll get a function decreasing from $+\infty$ for $y=0^+$ to $-\infty$ as $y \to 1$ as it should. Remember that the graph of the inverse function is obtained by mirroring the graph around the line $x=y$.