I suppose you don't have to constrain yourself to that specific data, so you could refer to the Census Bureau if you need more data. Moreover, you can search on Google for ''housing index methodology'' if you want references on how to build professional indexes. Personally, I think it would be great practice to replicate an index for your high-school project (remember to state in your work that it is not an original idea and to quote the sources). However, your teacher might think it is better that you come up with something yourself, even if not so elaborate. In that case, I would suggest you think about the following:
1) Which variables are useful, and for what they are used. Here are some guidelines:
- date: to see the evolution in time, but also allows you to aggregate by period and see: if you have seasonality, if the annual/quarterly count of transactions has increased or decreased, the time between transactions, etc.
- amount: by the amount, you can cathegorize within the three categories that you already have and possibly distinguish whether the real estate being bought/sold is cheap or luxury (this is a good indicator of the state of the economy)
- type: compare with indexes related to each one (confidence indexes, activity indexes, etc)
- state: some regions are more industrial, while others are more residential. Your index could indicate the market evolution in each state by pointing out the evolution of activity for each of the three categories;
- number of investor: some investors buy one house to live in and that is it, while others might do this as a business. If a number only appears once in the dataset, you probably have the first case, while if it appears many times you are probably facing an institutional investor.
2) An index should give you a quick assessment of the market over time. You should be able to compare two moments and tell whether the market is better or worse (should you invest in it or not), but you might also want to build something that gives more details on the market that you are observing.