Find Mean Value in a Column

Hi. I'm trying to compare the average baby's birth weights (bwt) among smoking mothers and non-smoking mother. There is a column 'smoke', where 1 represents smoking mothers and 0 represents non-smoking mom. Here is what I have:


And the answers I get are both NA which confuses me, can anyone please help? Thank you so much.

How does this work out?

mean(babies[babies$smoke==1, 'bwt'], na.rm = TRUE)
mean(babies[babies$smoke==0, 'bwt'], na.rm = TRUE)


  • Order of terms slightly changed;
  • na.rm = TRUE is a setting to ignore missing cases, just in case you have any.
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