Standard error of the mean
     
    
    Usage
    sem(x, missing.value = NA, return.missing.value = NA)
 
    
    Arguments
- x
- input vector 
- missing.value
- values that are missing 
- return.missing.value
- the value to return where there are missing values 
 
    
    Value
    The standard error of the mean of x
     
    
    References
    http://en.wikipedia.org/wiki/Standard_error