The first six months of Apple's (NASDAQ:AAPL) current fiscal year -- that's fiscal year 2018 -- have been quite strong. The company's revenue in that time was up 13.9% and its operating profit was up 12.6%.

That growth has been driven primarily by a 14% surge in iPhone revenue (thanks almost entirely to higher iPhone average selling prices), although the company's services, other products, and even iPad businesses grew by 24%, 37%, and 6%, respectively.

An iPhone 7 Plus in jet black.

Image source: Apple.

The Mac business has been something of a drag on the results, though, with revenue down 3% during the first half of fiscal year 2018.

With Apple soon set to report its financial results for the third quarter and provide guidance for the following quarter, here are some things that investors should keep their eyes on.

iPhone sales

iPhone shipments generally peak during the first quarter of a given fiscal year and decline over the following two quarters. Apple often sees a sequential bump in iPhone shipments during the fourth fiscal quarter of any year because that quarter often includes the first shipments of the new iPhones that launch in September.

Don't expect a deviation from that pattern when Apple reports its third-quarter results. It shipped 52.2 million iPhones during the second quarter of fiscal year 2018 so you should expect that shipment number to come down fairly significantly during the third quarter. 

Analyst Toni Sacconaghi with Bernstein Research revised his fiscal third-quarter iPhone shipment estimate to 38.8 million, which would represent a slight year-over-year decline in unit shipments (during the third quarter of fiscal year 2017, the company sold 41 million iPhones). That decline in unit shipments should, however, be offset by what's likely to be a year-over-year boost in iPhone average selling prices (thanks to sales of the iPhone X).

Keep an eye out for how iPhone unit shipments and average selling prices trended when Apple reports.

Guidance

While investors shouldn't ignore what Apple reports in the third fiscal quarter of the year, I suspect that people are going to be more interested in its guidance for the following quarter.

That guidance is especially important not only because Wall Street tends to care more about the future than the past, but because Apple's fiscal fourth quarter guidance will give investors insight into what Apple's initial expectations around its upcoming iPhone lineup.

I've been quite bullish on the trio of iPhones that are being prepared for this year, so any initial reads on how well Apple's expecting these new products to do in the market (analysts with BlueFin Research seem to think that Apple's planning for a robust product cycle).

The iPhone business makes up the bulk of its revenue -- 62% during fiscal year 2017 -- so the initial customer reception of the new iPhones is quite important and could give investors a sense of how this product cycle could play out.

Current analyst consensus calls for Apple to guide to $59.53 billion in sales (up 13.2% year over year) and for earnings per share of $2.65. Those will be the numbers that investors will benchmark Apple's guidance against -- if the company can beat them, that'd likely be good for the stock price. If it misses, that could put pressure on the shares.

Stock buyback activity

Back in early May, the board of directors approved the addition of $100 billion to the company's stock repurchase program. I'm interested in seeing how aggressively Apple bought back stock during the quarter and at what prices.

During its fiscal second quarter, the company bought back $32.85 billion worth of stock, which was up significantly from about $10 billion in the prior quarter and roughly $18 billion in the year-ago quarter.

Will Apple's newly implemented $100 billion share repurchase authorization embolden it to continue buying back stock at a furious pace? We'll see shortly.

Apple is set to report its fiscal third-quarter results and issue its fiscal fourth-quarter guidance on July 31 after the market closes.

Ashraf Eassa has no position in any of the stocks mentioned. The Motley Fool owns shares of and recommends Apple. The Motley Fool has the following options: long January 2020 $150 calls on Apple and short January 2020 $155 calls on Apple. The Motley Fool has a disclosure policy.