apparent vs actual attenuation

the potential attenuation listed for yeast, is that meant to be actual or apparent?

Apparent

How do you define the 2 terms? In my personal experience (which may or may not be representative) the yeast attenuation numbers are fairly close to the final product.

IME it all depends on wort fermentability. I've had 95% attenuation with US-05 (repitch) and very low mash temp.

william_shakes_beer said:

How do you define the 2 terms? In my personal experience (which may or may not be representative) the yeast attenuation numbers are fairly close to the final product.

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From Palmer's "How to Brew"

Attenuation

This term is usually given as a percentage to describe the percent of malt sugar that is converted by the yeast strain to ethanol and CO2. Most yeast strains attenuate in the range of 65 - 80%. More specifically, this range is the "Apparent" attenuation. The apparent attenuation is determined by comparing the Original and Final gravities of the beer. A 1.040 OG that ferments to a 1.010 FG would have an apparent attenuation of 75%.

(From FG = OG - (OG x %) => % att. = (OG-FG)/OG)

The "Real" attenuation is less. Pure ethanol has a gravity of about 0.800. If you had a 1.040 OG beer and got 100% real attenuation, the resulting specific gravity would be about 0.991 (corresponding to about 5% alcohol by weight). The apparent attenuation of this beer would be 122%. The apparent attenuation of a yeast strain will vary depending on the types of sugars in the wort that the yeast is fermenting. Thus the number quoted for a particular yeast is an average. For purposes of discussion, apparent attenuation is ranked as low, medium, and high by the following percentages:
65-70% = Low
71-75% = Medium
76-80% = High

BBL_Brewer said:

From Palmer's "How to Brew"

Attenuation

The apparent attenuation of a yeast strain will vary depending on the types of sugars in the wort that the yeast is fermenting.

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Like I said, it depends on mash temp/grain bill.

tre9er said:

Like I said, it depends on mash temp/grain bill.

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You're absolutely right, but that wasn't the question.

BBL_Brewer said:

You're absolutely right, but that wasn't the question.

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I was responding to william_shakes_beer, but didn't quote him

This is a pretty good video from Wyeast that I just watched last night. Around the 27 minute mark he talks about attenuation and even gives a tip how take a sample to get an idea how you wort is going to attenuate.