# Group by Gender_SNB and num_SNB, and calculate summary statistics av_data1_all <- s_complete %>% group_by(Gender_SNB, num_SNB) %>% summarise(mean_num_SNB = mean(num_SNB), sum_synch = sum(synch), count = n()) %>% ungroup()

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### indexing in R

- `(x)`: `x` is an object, in R, it is a vectore or data frame
- `x[i]`: `x` is a vector, `i` is an integer, which is the index of `x`
- `x[[i]]`: `x` is a data frame, `i` is an integer, which is the sequence number of `x`
- `x$i`: `x` is a data frame, `i` is a string, which is the variable name
- `x[i,j]`: `x` is a matrix, `i` and `j` are integers, which are the index of `x`
- `x[[i,j]]`: `x` is a data frame, `i` and `j` are integers, which are the sequence number of `x`
- `x$i[j]`: `x` is a data frame, `i` is a string, and `j` is an integer, which is the variable name and index
- `