The measures of the spread discussed above require observations on transaction prices and bid–ask quotes. Very often, however, quoted prices are either unavailable, unreliable or difficult to handle. As noted, quotes may not be binding or the spread may vary in the course of the trading day, so estimation of the spread based on transaction prices is usually preferred. Roll (1984) offers a very parsimonious model for estimating spread, using the time series of the prices at which trades were made. This model shows that when transaction costs consist of order processing costs only, i.e. in the absence of both adverse selection and inventory costs, the quoted spread coincides with the realized spread.

Roll demonstrates that it is possible to estimate spread by computing the autocovariance of transaction prices. Below we show how in the absence of transaction costs this auto-covariance is equal to zero, whereas with transaction costs, it is negative and a function of the spread. This allows us to estimate the spread by using transaction prices alone. This method proves useful when quoted bid and ask prices are not available.

When you get to the next stage—that is, when you have between $25,000 and $100,000 to invest—you can take a multilayered approach to your investing. I like the following simple five-part formula.

1. Cash. The first money you save should be marked for emergencies. This needs to be put someplace that is secure but easy to access, such as a home safe or a safe-deposit box. The amount you should keep for emergencies depends on your personal situation: how much you typically spend, how reliable your income is, and so on. As a rule of thumb, though, I’d recommend about 10 percent of your investable net worth. If you have $100,000, that would be $10,000.

2. Income-generating real estate. I recommend buying and flipping real estate for everyone, even beginners. If you start when you have less than $25,000 to invest and make a few deals, by the time your investable net worth hits $100,000, you should have a pretty active, nicely profitable second stream of income.

3. Side business(es). If you didn’t want to get involved in a side business when you had less than $25,000 to invest, you should consider it at this stage. You don’t have to risk a ton of money. Invest $10,000 conservatively in a business you understand and see where that takes you.