r - Subtract mean from individual response, across separate data frames -

my data structure

my experiment effect of condition (placebo or experimental) on power produced individuals during 3 sprint efforts, measured across 2 seperate sessions.

1 data.frame containes individual responses (individ) , structured as:

individ <- data.frame(subjectid = c(1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4),   sprint = c(1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3),   session = c(1, 1, 1, 2, 2, 2, 1, 1, 1, 2, 2, 2, 1, 1, 1, 2, 2, 2, 1, 1, 1, 2, 2, 2),   condition = c(placebo, placebo, placebo, placebo, placebo, placebo, exper, exper, exper, exper, exper, exper, placebo, placebo, placebo, placebo, placebo, placebo, exper, exper, exper, exper, exper, exper),   power = c(400, 250, 180, 500, 300, 450, 600, 512, 300, 500, 450, 200, 460, 254, 183, 540, 360, 420, 610, 514, 307, 508, 454, 201)) 

i obtained mean power each session via library(dplyr) , following code second data.frame:

averagenmt <- summarise(group_by(individ, session, condition), mean(power, na.rm = true)) 

my problem

i wish calculate deltas each individual's session. how subtract mean power, grouping session , condition, , adding in new column? expected output be:

deltascores <- data.frame(subjectid = c(1, 1, 2, 2, 3, 3, 4, 4),   session = c(1, 2, 1, 2, 1, 2, 1, 2),   condition = c(placebo, placebo, exper, exper, placebo, placebo, exper, exper),   deltapower = c(-11.1667, -11.6667, -3.16667, 11.16667, 11.66667, 189.1667, 2.166667)) 

do need use dplyr::mutate()?

assistance appreciated.

there may more elegant solution, need merge in summarized data able calculate on it. clean syntax via %>%

averagenmt  <- individ %>% group_by(session, condition) %>% summarize(mean_power= mean(power, na.rm=true)) ind2 <- merge(individ, averagenmt, by= c("session", "condition")) # name whatever want ind2 %>% group_by(subjectid, condition, session) %>% summarize(deltapower= mean(power) - mean(mean_power))  source: local data frame [8 x 4] groups: subjectid, condition    subjectid condition session deltapower 1         1   placebo       1 -11.166667 2         1   placebo       2 -11.666667 3         2     exper       1  -3.166667 4         2     exper       2  -2.166667 5         3   placebo       1  11.166667 6         3   placebo       2  11.666667 7         4     exper       1   3.166667 8         4     exper       2   2.166667