I have repeated measures of happiness for a sample of participants, and a single measure of satisfaction for each of the participants.
I want to predict satisfaction from the repeated measures of happiness. To do so, I want to create a new variable, called happiness.change, which measures the degree of change/trend in happiness from the first measurement to the last, for each participant (for example, a negative happiness.change if there is a decrease in happiness over time). Then I want to predict satisfaction from happiness.change.
Below (using R) is an excerpt from my data (a sample of 9 participants):
To create happiness.change I was advised to use the coefficients produced by either of these equations:
For example, running coef(model1) produces the following coefficients (column time):
And then connecting between satisfaction and the coefficients:
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I have several questions:
I realize this is quite long and these are a lot of questions, and so any help will be greatly appreciated. If anyone can answer even one of these questions, I will be very grateful.
Thanks!
I want to predict satisfaction from the repeated measures of happiness. To do so, I want to create a new variable, called happiness.change, which measures the degree of change/trend in happiness from the first measurement to the last, for each participant (for example, a negative happiness.change if there is a decrease in happiness over time). Then I want to predict satisfaction from happiness.change.
Below (using R) is an excerpt from my data (a sample of 9 participants):
Code:
ids <- c(rep(seq(1:5), each = 2), rep(6:9, each = 5))
time <- c(rep(1:2, 5), rep(1:5, 4))
happiness <- c(0.80,0.00,0.75,0.00,0.80,0.00,2.75,2.50,0.40,0.20,
3.80,0.40,0.00,0.20,3.40,3.00,2.60,3.40,3.80,0.00,
3.60,3.60,0.20,0.40,1.00,0.40,0.20,1.20,1.20,0.00)
satisfaction <- c(6,6,2,2,3,3,2,2,2,2,5,5,5,5,5,7,7,7,7,7,1,1,1,1,1,2,2,2,2,2)
data <- as.data.frame(matrix(c(ids, time, happiness, satisfaction),
nrow = 30,
ncol = 4,
dimnames = list(c(),c("id", "time",
"happiness", "satisfaction"))))
print(data)
# id time happiness satisfaction
# 1 1 1 0.80 6
# 2 1 2 0.00 6
# 3 2 1 0.75 2
# 4 2 2 0.00 2
# 5 3 1 0.80 3
# 6 3 2 0.00 3
# 7 4 1 2.75 2
# 8 4 2 2.50 2
# 9 5 1 0.40 2
# 10 5 2 0.20 2
# 11 6 1 3.80 5
# 12 6 2 0.40 5
# 13 6 3 0.00 5
# 14 6 4 0.20 5
# 15 6 5 3.40 5
# 16 7 1 3.00 7
# 17 7 2 2.60 7
# 18 7 3 3.40 7
# 19 7 4 3.80 7
# 20 7 5 0.00 7
# 21 8 1 3.60 1
# 22 8 2 3.60 1
# 23 8 3 0.20 1
# 24 8 4 0.40 1
# 25 8 5 1.00 1
# 26 9 1 0.40 2
# 27 9 2 0.20 2
# 28 9 3 1.20 2
# 29 9 4 1.20 2
# 30 9 5 0.00 2
Code:
require(lme4)
model1 <- lmer(happiness ~ time + (time | id), data = data)
require(nlme)
model2 <- lme(happiness ~ time, random = ~1 + time | id, data = data)
Code:
# $id
# (Intercept) time
# 1 0.9936158 -0.05991770
# 2 0.9739595 -0.05674569
# 3 0.9936158 -0.05991770
# 4 2.5539086 -0.31170766
# 5 0.8998610 -0.04478815
# 6 2.0446747 -0.22953078
# 7 3.2906564 -0.43059926
# 8 2.7120530 -0.33722798
# 9 1.0027053 -0.06138450
#
# attr(,"class")
# [1] "coef.mer"
Code:
coefs1 <- as.data.frame(unlist(coef(model1))[10:18])
satisfactionData <- reshape(data,
direction = "wide",
idvar = "id",
timevar = "time")[c(1,3)]
newData <- cbind(satisfactionData, coefs1)
colnames(newData) <- c("id", "satisfaction", "happiness.change")
print(newData)
# id satisfaction happiness.change
# 1 1 6 -0.05991770
# 3 2 2 -0.05674569
# 5 3 3 -0.05991770
# 7 4 2 -0.31170766
# 9 5 2 -0.04478815
# 11 6 5 -0.22953078
# 16 7 7 -0.43059926
# 21 8 1 -0.33722798
# 26 9 2 -0.06138450I have several questions:
- Running model2 generates the following error:
I don't remember where, but I read that running lme(happiness ~ time, random = ~1 + time | id, control = list(opt = "optim"), data = data) bypasses this error, and indeed it does. But what exactly does it do? Do the coefficients produced by using this model still represent the change in happiness over time?Code:Error in lme.formula(happiness ~ time, random = ~1 + time | id, data = data) : nlminb problem, convergence error code = 1 message = iteration limit reached without convergence (10)
- Running model2 (with control = list(opt = "optim")) produces slightly (very slightly) different coefficients than model1; why? What is the difference between the models?
- What would be the suitable method for testing the relationship between happiness.change and satisfaction? I tried cor.test(newData$satisfaction, newData$happiness.change), which produced the following results, but I'm not sure how to interpret them (they are significant in the complete dataset):
Code:Pearson's product-moment correlation data: newData$satisfaction and newData$happiness.change t = -0.72735, df = 7, p-value = 0.4906 alternative hypothesis: true correlation is not equal to 0 95 percent confidence interval: -0.7901064 0.4843018 sample estimates: cor -0.2650786 - As you can see, model1 produces negative coefficients for all the participants (model2 as well). Even for participants whose change over time is purely positive (for example, a participant with 2 happiness measures: time1 = 0 and time2 = 0.8; no such example in the sample above, but it is in my data).
This may be a problem, because I want to be able to distinguish between participants whose change in happiness is positive (happiness increases over time) and participants whose change in happiness is negative (happiness decreases over time); and then see whether there's a difference between these participants in their relationship between happiness.change and satisfaction. So my question is this: Is it statistically "legit" to divide my participants beforehand into groups based on their "raw" change in happiness (for example, I would label a participant with time1 = 0 and time2 = 0.8 as having a positive change), and then model the coefficient for these groups separately?
However, creating sub-groups this way may be difficult with participants with more than 2 measures of happiness, which brings me to my final question:
- If I understand correctly, model1 and model2 assume there is a linear change over time. However, I checked the whole sample (424 participants), and a cubic model (happiness ~ time) actually explains more of the variance in happiness. So I suppose a cubic change over time is more appropriate. How can I create a new variable reflecting such a change in happiness over time?
I realize this is quite long and these are a lot of questions, and so any help will be greatly appreciated. If anyone can answer even one of these questions, I will be very grateful.
Thanks!