CODEWITHSUNDEEP
rregressionlinear-regressionlmanova
J.marine
J.marine

Reputation: 69

Two-way ANOVA with weighted dependent variable

Trying to compare 3 independent populations across the years by the size of their individuals, I have this kind of data set:

year <- c(rep(2000,5),rep(2001,3),rep(2002,7))
region <- c(1,1,2,3,3,1,2,3,rep(1,3),rep(2,3),3)
size <- c(28,24,26,56,47,85,12,24,68,71,42,59,12,25,33)
count <- c(3,8,9,1,2,4,7,12,4,8,3,2,7,15,4)
df <- data.frame(year, region, size, count)

Which gives:

year region size count
2000      1   28     3
2000      1   24     8
2000      2   26     9
2000      3   56     1
2000      3   47     2
2001      1   85     4
2001      2   12     7
2001      3   24    12
2002      1   68     4
2002      1   71     8
2002      1   42     3
2002      2   59     2
2002      2   12     7
2002      2   25    15
2002      3   33     4

I want to make a 2-Way ANOVA: 

model.2way <- lm(size ~ year * region, df)  # example of code
anova(model.2way)

My issue is that the variable size is weighted by count: for each size, I have count number of individuals. I got millions of data and can't easily transform my data to have millions of size values.

Do you know a way to make a 2-Way ANOVA with this kind of weighted data?

Thanks in advance!

Upvotes: 2

Views: 3152

Answers (1)

Zheyuan Li
Zheyuan Li

Reputation: 73385

model.2way <- lm(size ~ year * region, df, weights = count)

From ?lm:

... when the elements of ‘weights’ are positive integers w_i, that each response y_i is the mean of w_i unit-weight observations ...

In other words, a weight of 2 means that case appears twice.

Upvotes: 2

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