The perils of tracking Apple’s supply chain ‘build plans’ by Philip Elmer-DeWitt @FortuneMagazine May 13, 2013, 3:38 PM EST E-mail Tweet Facebook Google Plus Linkedin Share icons Tim Cook visits an iPhone factory. Photo: Apple FORTUNE — In a note to clients Monday, Jefferies’ Peter Misek offered some intriguing numbers. Based on checks with his network of Apple AAPL suppliers, he believes that the company plans to build iPhones at the rate of 25 to 30 million by the end of June 25 to 50 million more by the end of September 60 to 65 million more by New Years and iPads at the rate of 20 to 25 million by the end of June (recently revised higher) 25 to 30 million more by the end of September 30 to 35 million more by New Years A word of caution before you pour those numbers into a spreadsheet: Data points from individual suppliers are notoriously unreliable, as Tim Cook likes to remind analysts. And as we have seen, such estimates can go down as well as up. One memorable case in point: In Nov. 2011 DigiTimes reported that Apple had “slashed” orders for iPhone 4S parts 10% to 15% — a report that generated a flurry of doomsday headlines (Uh-Oh: Apple Said To Cut Orders To Asia Suppliers On iPhone 4S Problems” from Business Insider’s Henry Blodget) and persuaded many on Wall Street that Apple was headed for disappointing Christmas sales. As it turned out, the company shipped a record 37 million iPhones that Christmas quarter, up 128% year over year.