SAS Employee

## Calculation of Logit(Y)

Hi All,

I need your help in the calculation of Logit(y)

Logit(y) = -300 + 0.5 * x + 0.3 * y

x=30

value of y is unknown

Thanks

1 ACCEPTED SOLUTION

Accepted Solutions
SAS Super FREQ

## Re: Calculation of Logit(Y)

Please recheck your equations. For that value of x, you are asking for the value y such that

logit(y) = -285 + 0.3*y;

Did you really intend to have y on both sides of the equation?

If so...

For the logit function to be defined, y is in (0, 1). The value logit(y) approaches minus infinity as y ->0, so we are looking for a tiny value of y. Therefore we can effectively ignore the term that includes y on the right and approximate the solution by

logit(y) = -285

or

y = logistic(-285) = 1.68E-124.

If y represents the probability of some event, this calculation shows that the probability is effectively 0.

4 REPLIES 4
Super User

## Re: Calculation of Logit(Y)

You already have the formula, so where's the problem? In code, you just need this:

``````data want;
set have; * provides values for y;
x = 30;
result = -300 + 0.5 * x + 0.3 * y;
run;``````
Super User

## Re: Calculation of Logit(Y)

It seems you are looking for a root of function.

@Rick_SAS  wrote some blogs about it .

SAS Super FREQ

## Re: Calculation of Logit(Y)

Please recheck your equations. For that value of x, you are asking for the value y such that

logit(y) = -285 + 0.3*y;

Did you really intend to have y on both sides of the equation?

If so...

For the logit function to be defined, y is in (0, 1). The value logit(y) approaches minus infinity as y ->0, so we are looking for a tiny value of y. Therefore we can effectively ignore the term that includes y on the right and approximate the solution by

logit(y) = -285

or

y = logistic(-285) = 1.68E-124.

If y represents the probability of some event, this calculation shows that the probability is effectively 0.

SAS Employee

## Re: Calculation of Logit(Y)

Thank you - for the explanation.

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