variance.py

# Variance for a discrete random variable
class VarianceCalc(object):
    # This function will take in two lists, one being the
    def calculate_variance(self, variable_list, probability_list=None):
        # attempt to calculate variance
        try:

            # if true, dealing with single list of values
            if probability_list is None:

                # When presented a null set, spit back a null value
                if len(variable_list) == 0:
                    return None

                # When presented a 1 value set, the variance is 0
                elif len(variable_list) == 1:
                    return 0

                # When presented a multi value set, calculate the variance
                else:

                    # start by calculating mean of the set

                    # calculate sum of set
                    sum = 0
                    for i in variable_list:
                        sum += i

                    # divide by number of values to get mean
                    mean = float(sum) / len(variable_list)

                    # Iterate through set to get sum of square differences
                    sum_square_diff = 0

                    for i in variable_list:
                        sum_square_diff += pow(( i - mean ), 2)

                    # Divide by sample size -1 to get variance
                    variance = round(sum_square_diff / (len(variable_list ) - 1), 3)
                    return variance
            # Else dealing with a discrete random variable
            else:
                # Not yet implemented
                return None
        except:
            # Print a general error statement until more description is available
            print "An error has occurred."