Smart way to identify strange observations?

[Rachel Sleeps]

Hi, I am using Stata 13 and I have some sales data from a few stores. I scaled the sales data by store size and plotted it over time. I just wanted to get a feel for the data. It looks a bit funny (see the graph below).

There are a few stores that do not have any (what seems like) seasonal fluctuation over time. I wonder if there is any smart way to identify these stores systematically? My initial idea was to identify them by variance. Here is what I did:

gen year_month = mofd(Date)
    format year_month %tm
    

        
*scale monthly sales by size
gen monthly_sales_by_size = monthly_sales/Size
        
        

        
*Some graphs to get a feel for the data
line monthly_sales_by_size year_month
    
    
*get variance by store
bysort Store: egen sales_variance = sd(monthly_sales_by_size)
sum sales_variance
    
drop if sales_variance <0.005
line monthly_sales_by_size year_month, legend(size(medsmall))

But that is such a crude way of doing it. Any idaes? I think it is a very interesting problem

Thank you in advance! /R

[William Lisowski]

I think that you don't actually have a problem with your stores, but rather a problem with your plot.

Are you inferring lack of seasonality from the straight lines that run all the way from the left edge to the right?

The problem is that what you wanted was a plot that overlaid several stores on the same plot. What you have is a plot that is one long line for all the stores. The lines you see as running straight from left to right are actually running from the (right) end of one plot to the (left) beginning of the next.

Without seeing your plot command I will not speculate further, but I think you're missing a by() option to produce separate plots for each store.

[Nick Cox]

sort Store year_month  
 line monthly_sales_by_size year_month, legend(size(medsmall)) c(L)

would probably remove the spurious connections. Otherwise what's especially odd? I would probably also try

gen Month = month(Date) 

line monthly_sales_by_size Month, by(Store) c(L)

[Rachel Sleeps]

Wow....what a rookie mistake....Thank you both!