def get_factor_names(factor_count):
    """Returns a list of factor variable names up to the count.

    Example:
        >>> get_factor_names(3)
        ["X1", "X2", "X3"]
    """
    # Design-Expert uses A/B/C for variable names, switching to A'/B'/C'
    # and A"/B"/C" after 25 or 50 variables (I is not used)
    # unfortunately the prime and double prime (single/double quote) characters
    # are not really suitable for strings
    # use X1/X2/X3 instead, which is common in academic material (see the NIST
    # pages,for example)
    # return [get_var_name(i) for i in range(factor_count)]

    return ["X" + str(i+1) for i in range(factor_count)]