by including NA in " " we transform it into a real missing value in an R factor vector
```
HIV_coded$bisexual<- factor(replace(HIV_coded$bisexual, HIV_coded$bisexual == "NA", NA))
```
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Sometimes when you are running a regression model with variables that have different lengths, r will prompt with the following error message...
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Interesting read: http://sia.webpopix.org/nlme.html
