code: def calc_t(w1, w2, weight_fn=None):
      
      if not w1 or not w2:
          return 0
  
      s1 = analyze(w1, weight_fn)
      s2 = analyze(w2, weight_fn)
      delta_s = s2['avg'] - s1['avg']
  
      if delta_s == 0:
          return 0
      if s1['variance'] == 0 and s2['variance'] == 0:
          return float('inf')
  
      return delta_s / (((s1['variance'] / s1['n']) + (s2['variance'] / s2['n'])) ** 0.5)
docs: Perform a Students t-test on the two sets of revision data.
  
      See the analyze() function for a description of the `weight_fn` argument.