Small multi family valuation

[David A Love]

I am currently reading a book written in 2006 about investing in duplexes, triplexes, and quads. He mentions "Gross Rent Multiplier", a method of determining the value of a small multi family property based on the income generated by the property, similar to valuation of larger commercial property. I have read ALL the books and listened to ALL the podcast for years, and nobody has ever mentioned GRM. So, why have I never heard of this until now? Do brokers, appraisers, landlords, and investors still apply this method to small multi family? Or is comparable sales the preferred method for this niche?