Introduction

City Bike is a bike share system serving New York City and Jersey City which was launched in 2013. Riders can get access to Citi bikes at hundreds of stations in New York City. The system operates 24 hours a day, 365 days a year (https://en.wikipedia.org/wiki/Citi_Bike).
Since Citi Bike services are widely used by New Yorkers, it is important to understand the riding patterns in order to provide better services. For example, analyzing the user types will facilitate the allocation of Citi Bike stations based on the demographic information of different neighborhoods. In this Citi bike analysis, I will examine the day and night ridership. Specifically, I am interested to find out if older people (>=40) are less likely to use Citi bikes at night (19:00-5:00) compare to younger people. 

Data

The data used is Citi Bike trip data which is published on the Citi Bike website (https://www.citibikenyc.com/system-data).  Variables included in the trip history data are listed below.
In the analysis, Citi bike trip data of March 2015 was used. Data of July 2015 was selected to verify the analysis results. Data was cleaned to only include the variables needed which were the start time in datetime format and age of riders.
First, the distribution of the number of trips during 24 hours was plotted. The riders were separated into two age groups with the threshold of 40 years old. Figure 1 shows the absolute counts of the number of trips and Figure 2 shows the normalized results. Uncertainties were calculated using Poisson statistics. Figure 2 shows that the two groups of riders do have different patterns when using Citi Bikes. Younger riders tend to use the bikes more at night hours. We need to include statistical tests to test the hypotheses.