Conclusion

Based on the outcome of the U test, I'm able to reject the null hypothesis and assert that there is a significant difference between the average trip duration of subscriber and customer. To strengthen the analysis, I chose another data set (January 2015)  to do the same test and I got the same result as above.  However, the analysis does not indicate the scale of the difference, whether the subscriber has longer trip duration than customers or they actually has shorter trip duration compare to the customers. Further experiment can be conduct to see the scale of difference between user type of Citi Bike which could better inform the user base improve the membership mechanism.