So I have data from a randomized blind trial of 1mg of nicotine gum on dual n-back working memory scores; I analyzed them as usual with a t-test and found a small increase in means but a large increase in standard deviations on a f-test! Strange. I also have data for each day on mood/productivity that day on a 1-5 scale.
I wondered: is nicotine following an inverse U-curve, where it causes higher scores on the worser days (1-3) and lower scores on the better days (3-5)? I look around and it seems I want a multinomial logistic regression comparing the placebo & active days.
I enter the data & load mlogit
:
nicotine <- read.table(stdin(),header=TRUE)
day active mp score
20120824 1 3 35.2
20120827 0 5 37.2
20120828 0 3 37.6
20120830 1 3 37.75
20120831 1 2 37.75
20120902 0 2 36.0
20120905 0 5 36.0
20120906 1 5 37.25
20120910 0 5 49.2
20120911 1 3 36.8
20120912 0 3 44.6
20120913 0 5 38.4
20120915 0 5 43.8
20120916 0 2 39.6
20120918 0 3 49.6
20120919 0 4 38.4
20120923 0 5 36.2
20120924 0 5 45.4
20120925 1 3 43.8
20120926 0 4 36.4
20120929 1 3 43.8
20120930 1 3 36.0
20121001 1 3 46.0
20121002 0 4 45.0
20121008 0 2 34.6
20121009 1 3 45.2
20121012 0 5 37.8
20121013 0 4 37.2
20121016 0 4 40.2
20121020 1 3 39.0
20121021 0 3 41.2
20121022 0 3 42.2
20121024 0 5 40.4
20121029 1 2 41.4
20121031 1 3 38.4
20121101 1 5 43.8
20121102 0 3 48.2
20121103 1 5 40.6
library(mlogit)
Nicotine <- mlogit.data(nicotine,shape="wide", choice="mp")
mlogit(score ~ (active + mp)^2, Nicotine)
Error in solve.default(H, g[!fixed]) :
Lapack routine dgesv: system is exactly singular
Calls: mlogit ... mlogit.optim -> as.vector -> solve -> solve.default
The error also happens even with the simplest call I can think of:
mlogit(score ~ active, Nicotine)
Error in solve.default(H, g[!fixed]) :
Lapack routine dgesv: system is exactly singular
Calls: mlogit ... mlogit.optim -> as.vector -> solve -> solve.default
Reading the documentation for mlogit
didn't much help, and look at the other questions having the same error, they're different enough I can't tell whether they apply or not.
Thank you for your assistance.
Best Answer
You don't want multinomial logit as your dependent variable is a score that is nearly continuous. I would start by plotting the data e.g. with
which doesn't reveal any obvious pattern.
Then you could look at a linear model:
(this is equivalent to the t-test you ran) which shows a small and nonsignificant difference. Plotting m1 doesn't reveal anything particularly interesting to my eyes, either. You say you found large differences in variances but
shows the difference to be not all that large (and the stripchart shown above agrees).
Then you could add mp to the model:
which also shows only very small differences and a miniscule $R^2$
There's probably other things you could do, but it looks like there is not much to find here.