[R] Strange variable names in factor regression

[Michael Dewey]

Comment in in-line below

On 09/05/2024 13:09, Naresh Gurbuxani wrote:
> 
> On converting character variables to ordered factors, regression result
> has strange names. Is it possible to obtain same variable names with
> and without intercept?
> 
> Thanks,
> Naresh
> 
> mydf <- data.frame(date = seq.Date(as.Date("2024-01-01"),
> as.Date("2024-03-31"), by = 1))
> mydf[, "wday"] <- weekdays(mydf$date, abbreviate = TRUE)
> mydf.work <- subset(mydf, !(wday %in% c("Sat", "Sun")))
> mydf.weekend <- subset(mydf, wday %in% c("Sat", "Sun"))
> mydf.work[, "volume"] <- round(rnorm(nrow(mydf.work), mean = 20, sd =
> 5))
> mydf.weekend[, "volume"] <- round(rnorm(nrow(mydf.weekend), mean = 10,
> sd = 5))
> mydf <- rbind(mydf.work, mydf.weekend)
> 
> reg <- lm(volume ~ wday, data = mydf)
> ## Variable names as expected
> coef(reg)
> (Intercept) wdayMon wdaySat wdaySun wdayThu wdayTue
> 21.3846154 1.3076923 -12.0000000 -12.9230769 -1.9230769 -0.6923077
> wdayWed
> -1.6153846
> 
> reg <- lm(volume ~ wday - 1, data = mydf)
> # Variable names as expected
> coef(reg)
> wdayFri wdayMon wdaySat wdaySun wdayThu wdayTue wdayWed
> 21.384615 22.692308 9.384615 8.461538 19.461538 20.692308 19.769231
> 
> # Ordered factors for weekday sequence
> mydf$wday <- factor(mydf$wday, levels = c("Mon", "Tue", "Wed", "Thu",
> "Fri", "Sat", "Sun"), ordered = TRUE)
> 
> reg <- lm(volume ~ wday - 1, data = mydf)
> # Variable names as expected
> coef(reg)
> wdayMon wdayTue wdayWed wdayThu wdayFri wdaySat wdaySun
> 22.692308 20.692308 19.769231 19.461538 21.384615 9.384615 8.461538
> 
> reg <- lm(volume ~ wday, data = mydf)
> # Strange variable names
> coef(reg)
> (Intercept) wday.L wday.Q wday.C wday^4 wday^5
> 17.406593 -12.036715 -4.968654 -1.852819 3.291477 4.263642
> wday^6
> 2.591317


Yes, that is what ordered is supposed to do, fit polynomial contrasts.
When you remove the intercept that breaks it so you get a different fit, 
in fact the same as you got when it was not ordered.

Michael

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-- 
Michael