Treasury Bills.
The 3-month Treasury bill interest rate is watched by investors and economists. Here's a scatterplot of the 3-month Treasury bill rate since 1950:
![1901_graph 2.jpg](https://secure.tutorsglobe.com/CMSImages/1901_graph%202.jpg)
Clearly, the relationship is not linear. Can it be made nearly linear with a re-expression? If so, which one would you suggest? If not, why not?