obs   year       calls   
   1      50        4.4  
   2      51        4.7  
   3      52        4.7  
   4      53        5.9  
   5      54        6.6  
   6      55        7.3  
   7      56        8.1  
   8      57        8.8  
   9      58       10.6  
  10      59       12.0  
  11      60       13.5  
  12      61       14.9  
  13      62       16.1  
  14      63       21.2  
  15      64      119.0  
  16      65      124.0  
  17      66      142.0  
  18      67      159.0  
  19      68      182.0  
  20      69      212.0  
  21      70       43.0  
  22      71       24.0  
  23      72       27.0  
  24      73       29.08     4.38     4.90