Dear all,

Using Stata 15, we are analyzing the effect of age (
age is from 16 years to 80 years)
, period (year of the Survey; time gab 3 years) and cohort (birth year in 5-year cohorts) [APC-models; e.g. Yang & Land, 2006] on the percentages of different beverages (e.g. beer, wine, spirits etc.) of alcohol overall consumption, e.g. 40% of all alcohol consumed was beer. We are using individual data, Overall 50.000 persons, 7 surveys. Therefore, we use a fractional multinomal logit model (fmlogit; Buis, 2008). Nevertheless, period and cohort are coded as factors while age is used as a restriced cubic spline using rc_splines for flexibility reasons.
The main aim is calculating predictions (margins) for age, period and cohort and marginal effects for interpretation purposes (since the parameters in the model are difficult to interpret).
The main problem is getting predictions (margins) or marginal effects for the age-splines.

Short example:

rc_spline age, nknots(5)
fmlogit beer wine spirits, eta(i.period i.cohort _Sage_1-_Sage_4)
*cohort-effects
margins i.cohort, at( (asbalanced) _factor (mean) _continuous) force predict(outcome(beer))
*period-effects
margins i.period, at( (asbalanced) _factor (mean) _continuous) force predict(outcome(beer))
*age effects
margins, at( _Sage_1=(16(1)80) (asbalanced) _factor (mean) _continuous) force predict(outcome(beer))
*marginal effects
margins, dydx(*) predict(outcome(beer))

Is this the "right way" to get "age effects" - margins for age? Because postrcspline is not applicable with fmlogit and only displays the results in a graph; ist here another way to include all spline variables to get margins?
How to get
margin effects for age because postrcspline is not applicable and dfmlogit is not applicable for Stata 15?; or is ist even possible to get soething like marginal effects for a continous variable or spline?


Thanks in advance.



Reference:
Yang, Yang and Kenneth C. Land. 2006. ‘‘A Mixed Models Approach to the Age-Period-Cohort Analysis of Repeated Cross-Section Surveys: Trends in Verbal Test Scores.’’Sociological Methodology 36:75-97