returns a weighting from 0 to 1 based on the sources.
        Due to fractions multiplying resulting in very small 
        numbers, adding 0.5 to bias calculations which means
        actual range is 0.5 -> 1.5 (still testing)
        
        Note - range should be from 0 - > 1

def _calculate_bias(self):
        """
        returns a weighting from 0 to 1 based on the sources.
        Due to fractions multiplying resulting in very small 
        numbers, adding 0.5 to bias calculations which means
        actual range is 0.5 -> 1.5 (still testing)
        
        Note - range should be from 0 - > 1
        
        """
        for m in self.metadata:
            for b in self.bias_details:
                if  b[0] == 'bias-' + m['label'] + '.csv':
                    l_bias = 1.000
                    try:
                        l_bias = float(b[2]) + 0.5
                    except:
                        lg.record_process('bias.py','ERROR converting bias value to float: ' + str(b))
                    self.bias_rating *= l_bias