why would lm use time index what is the function of plm why does an ols need unique time pairs? what model doesn't require a time indices?? why a model does not require time indices does a fixed effect ols need a time variable? why is plm used in r why does lm function need unique time why does r require time indices why does a fixed effect ols require unique time elements? why does a fixed effect ols need unique time indices if lm is used to create an example of an ols the model fails why do ols models require unique time variables why is plm a function why does an ols model need unique time indices? why does a fixed effect model require unique time indices r how to create a time indexed which function does invokable time elements require? why do ols models need unique time indices what is the lm function