How did a Romanian blogger named Horace Dediu manage to predict Apple’s (AAPL) quarterly results over the past three quarters more accurately than anybody else in the business?

The secret is in the spreadsheet above, posted on his Asymco website Thursday morning.

What Dediu does, as he will patiently explain to anyone who asks, is carefully monitor the growth rates of each of Apple’s lines of business. He starts with the assumption those rates will persist, and then adjusts his estimates as events occur — like a new model or rising component costs — over the course of a quarter.

“The major changes from the last quarter,” he writes, “were the acceleration in software sales and the return to growth in iPod sales. iPod benefited from the continuing shift to iOS as the mix is now 50% touch in the portfolio…

“Overall the growth is both wide and deep. I’m most impressed with the iPod as it was a surprise to hear such a high mix of touch.”

Below: Dediu’s chart of the top and bottom lines plotted over 16 quarters. The color bands correspond to the A to F scoring system used above.

See also:

• The new king of Apple analysts
• Apple analysts: Who’s getting better, who’s getting worse

[Follow Philip Elmer-DeWitt on Twitter @philiped]