Dallas Cowboys : biggest year-to-year improvement/decline
based on regular season win percentage:

                            Change in 
                              win %
Improvement: 1989 -> 1990     0.376   (0.062 -> 0.438)
Decline:     2009 -> 2010    -0.313   (0.688 -> 0.375)
Last Year:   2010 -> 2011     0.125   (0.375 -> 0.500)