Outputting Maximum and Minimum Values

#!/usr/bin/python3

m1980 = 14.75
m1990 = 14.43
m2000 = 14.13
m2012 = 14.30

s1980 = 7.39
s1990 = 6.67
s2000 = 5.60
s2012 = 3.52

def march_frac_years(w, x, y, z):

        print '2012 vs. 2000s'
        m2012_1980 = z/w
        print '  March coverage fraction ' + str(m2012_1980)

        print '2012 vs. 1990s'
        m2012_1990 = z/x
        print '  March coverage fraction ' + str(m2012_1990)

        print '2012 vs. 2000s'
        m2012_2000 = z/y
        print '  March coverage fraction ' + str(m2012_2000)

        max_winter = min(m2012_1980, m2012_1990, m2012_2000)
        print 'Max winter coverage loss ' + str(max_winter)

march_frac_years(m1980, m1990, m2000, m2012)

def frac_years(numer, denom, ctr, month):
    if ctr < 3:
        result = numer/denom
        print '  %s coverage fraction %f' % (month, result)
        return result

    if 3 == ctr:
        max_winter = min(numer, denom, month)
        print 'Minimum winter coverage loss ' + str(max_winter)

lits_list = ['2012 vs. 1980s', '2012 vs. 1990s', '2012 vs. 2000s']
march = [14.75, 14.43, 14.13, 14.30]
sep = [7.39, 6.67, 5.60, 3.52]
march_results=[]
sep_results=[]
for ctr in range(3):
    print lits_list[ctr]
    result=frac_years(march[ctr], march[3], ctr, "March")
    march_results.append(result)
    result=frac_years(sep[ctr], sep[3], ctr, "September")
    sep_results.append(result)
    print
frac_years(march_results[0], march_results[1], 3, march_results[2])
frac_years(sep_results[0], sep_results[1], 3, sep_results[2])