Counting under uncertainty
But bicycle riding is a great example to use in the proper use of data from the American Community Survey. Ideally, the ACS should only be used to measure characteristics of a given population, and not be relied on to be the reference for the count of any particular group of the population. That being said, many do use the ACS to report counts of many things,and even I fall into that trap on occasion, but I really ought not to.
What am I talking about? The ACS is a sample survey program that for small populations, either in small geographies or small subgroups of a larger population, you really have to be aware of sample error. The program is not designed to be the reference count of the population and wonks out there realize that population counts from the Census Bureau's population estimates program are not the same as what you will get out of the ACS in most cases. Some like to pick and choose which data works best for them and you really need to be careful if you want to be objective.
So I ramble... counting bicycle usage in Pittsburgh...
So a routine metric in the city of Pittsburgh is bicycle usage. A common question in the ACS and other programs is to ask your means of transportation to work, i.e. commuting. Commuting is not the only reason to travel by bicycle. If you are a student getting to class, or shopping, or just riding to ride, you are not traveling to work and thus not captured in any data about commuting technically. But on the focused question of the trend in bicycle commuting in the city of Pittsburgh, you get a much better picture of the trends if you look collectively at the results from the American Community Survey over multiple years. Relying on any one year of data, and just using the point estimate itself, really will throw you off. Some years new data will imply a big jump, while some years will show decline on occasion.
So here is my graphic of the ACS estimates of the number of bicycle commuters residing in the city of Pittsburgh. Note the actual estimate is not shown at all and the bars represent the range from the estimate + or - the reported margin of error. You get a far different feel for the trend when you look at the data like this, but for sure an upward trend.If I had just plotted the estimates by themselves (granted something the world does often, to include me) you give rise to an illusion of accuracy that is not there. For those who are not ACS data users, basically the program reports data for different time periods. Some data is reported as an annual (1-year) average. Some data is reported over a 3-year average and some data is reported over a 5-year average. Here are all relevant data on the number of bicycle 'commuters' in all of the ACS datasets.