In this clip from the Industry Focus: Tech podcast, Dylan Lewis and Nathan Hamilton talk about how to use iPhone sales as a metric to measure Apple's (NASDAQ:AAPL) quarterly earnings success.

Find out how the company did in its most recently reported quarter in terms of iPhone sales, what factors most affect the revenue in its top product category, and the proper time frame to consider when looking at comparable iPhone sales.

A transcript follows the video.

This podcast was recorded on April 29, 2016. 

Dylan Lewis: The first of those things you wanted to focus on was iPhone sales. Why don't we start there? For the most recent quarter, 51.2 million units sold. They still clock in at roughly two-thirds of their revenue, so that is the big driver for them, and, obviously, probably the culprit when you look at this down year-over-year revenue growth. So, what did we see with that, and what can investors make of it?

Nathan Hamilton: We were talking about it a little bit before. You almost have to look at where Apple is in the upgrade cycle and put that into context. When Apple went from the 5S to the 6, different change in form factor, everything changed, and they had blockbuster sales. I can't remember the exact numbers at the time, but I recall 40% year-over-year growth during some quarters when the 6 came out. Where we are now, essentially, the iPhone 6SE, essentially a 5 form factor with better guts to it, better processors? That was essentially on sale starting in April. It's not even reported in the most recent quarter for Apple. 

You have to look at it that way. It's pretty much expected that iPhone sales will come down a little bit when there isn't a huge upgrade, and when there isn't a change in the form factor altogether. You mentioned a good point, you almost have to look at Apple in two-year cycles, and say, "OK, what's happening here?" Because, the longer-term side of it, Apple is still adding territories. They're still growing in a lot of different, important countries. The U.S., of course, isn't doing much for actual iPhone unit sales. The companies pretty much trade market share. But the longer-term story, you see a slightly upward trending trajectory for iPhones over time, but from quarter to quarter, definitely not.

Lewis: I totally agree. It's tempting to look at companies' comps year-over-year. But when you have this very clear product cadence of the new numerical issue, and then the new numerical issue S, and that's the year-to-year march. It is, in some ways, not really fair to compare numbers year to year. It's almost more of a two-year comp cycle.

Hamilton: Yeah. And to put that in context, it's not just a matter of, OK, things will still go great for Apple. If you look at previous iPhone releases, they did have the quarter-over-quarter growth, even in down times where there wasn't a fresh model coming out. It's not the most favorable, but I don't think it's anything, if you're looking to Apple as a somewhat safer stock to say, "OK, I'm out of it, hit the sell button, I'm done, Apple."

Lewis: Yeah, and one other note with how particularly rough the comps that they were going up against were -- they actually had a lot of supply constraints when they issued the 6 to begin with. So, a lot of the demand for the 6 bled into this quarter. It was originally launched in fiscal Q1, but because of pent-up demand and supply issues, a lot of that wound up trickling into Q2. So that inflated what was already incredible demand because of the new form factors and a lot of the new things they brought to that line.

Dylan Lewis owns shares of Apple. Nathan Hamilton owns shares of Apple. The Motley Fool owns shares of and recommends Apple. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.