Getting the most affordable interest rate for a mortgage requires a substantial amount of time and research.

Not only must you become familiar with rates at specific lenders, but you need to expand your knowledge of mortgages overall. Many factors come into play when lenders determine rates, and understanding how to properly compare them can help you make the most informed decision.

Get a baseline with national averages

Before calling lenders for a rate quote, hop on the Web and visit sites that provide national averages for mortgage rates. Freddie Mac, a government-sponsored enterprise that guarantees home loans, publishes a weekly report on average rates each Thursday. The details include information about 30- and 15-year fixed-rate loans as well as some adjustable-rate mortgages. and some online real estate marketplaces such as Zillow also publish rate information.

Different loan types have higher or lower rates

Just as there are various home inspection services tailored to different home needs, there are various mortgage types. Keep in mind that certain home loans have higher rates on average compared to others due to the loan terms. 

FRMs tend to have higher interest because the rate remains consistent throughout the loan term. If, for instance, you get a rate of 4.14 percent, you'll have that rate until you've repaid the loan or refinanced. ARMs, on the other hand, usually have a lower rate compared to FRMs because they adjust - usually to a higher rate - after a certain amount of time.

Economic changes cause rate fluctuations

Interest rates can change daily or even by the hour depending on the state of the economy. One positive or negative report from a government agency can send rates up or down. Unemployment data, for example, typically has an effect on interest rates.

If you call a lender one day, the quote may be different if you submit another request the following day. Given these fluctuations, call the lenders in your roster of choices within a short time to get an accurate comparison.

Final rates are based on your choices, financial history

Remember that quotes are not a guarantee. There's a chance the final interest rate you receive will be higher or lower than the estimate depending on your credit, employment and income history, among other factors. Also, certain choices you make, such as whether you decide to pay points, can alter your interest rate.