Conclusions
The results of analysis match the idea that the post-90s are less likely than pre-90s to choose biking for commuting. And the conclusions of July and December are the same so that our work is robust to sensonality.
For testing its significance, we calculate the Z-statistics as 30.98 in July and 28.24 in December, we got the corresponding p-value < 0.0002, which is smaller than my chosen α=0.05. So we can reject the Null Hypothesis and my conclusion is statistically significant (by a lot!)
Strength: We got a more reliable conclusion by analyzing two significantly different months in one year.
Weakness: We did not pick gender as a critical character given that different genders make different choices ( proved in FBB's instruction). If the majority of post-90s users are females, our work is a simple duplication of FBB's and lose its sense.