# Empirical logit transformation on percentage data

I have already used the logit transform on my outcome variables (which are displayed in percentages). However, this obviously gives me -INF values and since my data includes a lot of zeros in some instances, this makes it hard to analyse.

I have now tried an empirical logit transform, adding the smallest non-zero promotion to the numerator and denominator of my variables to remove the -INF values (as suggested in http://www.esajournals.org/doi/abs/10.1890/10-0340.1).

However, now my data are very non-normal again. I have tried experimenting with error terms to add to the logit transform but since have had no luck.

Is there any way I can find a value to add to my transformation to ensure normality?

• A worthy option for your consideration is a generalized linear model. Please search our site for threads on GLMs. If you still want to transform the response, then search for threads about transformations, logarithms, and regression: many of them explicitly discuss whether and how to add a "start value" to the data before re-expressing them. – whuber Jul 30 '14 at 19:13
• If your outcome variables are 'displayed in percentages' this suggests that they aren't originally percentages. Presumably they are counts. @whuber is suggesting starting instead with a logistic (or multinomial logistic) regression model, for which even conditional normality is not a requirement. – conjugateprior Jul 30 '14 at 22:36
• Thanks for these. However, although I do have the raw counts, the particular research I am carrying out means that I would expect that the raw counts would increase with my predictor variables. Therefore, percentages are giving me a more reliable measure in my analyses. – user3237820 Jul 31 '14 at 21:27
• As per @whuber... Are you sure this isn't a binomial process? I would pick a trials count for each record and model it as success and failures. glm binomial supports this. If it isn't binomial in nature, perhaps try inverse hyperbolic sine transformations worthwhile.typepad.com/worthwhile_canadian_initi/2011/07/… – Chris Dec 11 '17 at 18:27

I've had luck with setting epsilon to half of the smallest non-zero value and replacing all 0 values with epsilon and all 1 values with 1-epsilon. Then apply the logit transformation.

This method keeps the original form of the logit transformation, but allows 1 and 0 to be transformed to values that match the overall shape of the intended transformation (note the black dots in the figure at raw=0 and 1). In particular, it preserves the quality that 0.5 is transformed to 0, and the rest of the values are symmetric.

On the other hand, adding the smallest non-zero value as described in the paper changes the shape of the curve and destroys the symmetry.

• Welcome to our site! This thoughtful, well-illustrated post is a useful contribution. – whuber Jul 30 '14 at 19:14
• Thanks! I will give this a go, although I have been experimenting with zero/one inflated beta regression too. – user3237820 Jul 31 '14 at 21:28

One approach, which would solve the problem you are having, is to use a robust regression method on the raw, untransformed values. For example, in R, you could do the following:

example = data.frame(outcome = c(0,0,0.3,0.7,1),
predictor = c('left','left','left','right','right'))
m = glm(outcome ~ predictor,example,family=quasibinomial())
summary(m)