How to remove NA as a level

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))
```

Introduction to the Analysis of Survival Data in the Presence of Competing Risks

 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4741409/

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