February 21, 2013

Weekly Language Usage Tips: forming possessives & accuracy and precision

Posted in accuracy/precision, apostrophe, apostrophe after s, apostrophes and plurals, possessives at 6:13 am by dlseltzer

Tip 1: Showing possession and that pesky apostrophe

A reader writes:

Quick question.  Chris is the name of the care manager for our Online Treatments Trial.

We have a discussion board on our website where under his name we post a ‘featured discussion’ that we change each week for all to view and comment.

Is it correct to name the featured discussion board under  the heading “Chris’ Corner”  or “Chris’s Corner”? Thanks!

 We’ve talked about forming possessives before, but it keeps coming up, so let’s take another whack at it. Before we get into the whys and wherefores, let me answer the reader’s question. It’s correct to name the discussion board ‘Chris’s Corner.’ It used to be the other way (just adding an apostrophe if the word ends in ‘s’), but for most of the world, that convention has changed.

This is the current rule for making a singular word possessive: Add an ‘apostrophe s.’ That’s pretty much it. It doesn’t matter if the word ends with an ‘s’ or not. And it makes sense, too. That’s the way we pronounce it.

 The class’s picture came out very well this year.

 We pronounce ‘class’s’ with two syllables not one, so we might as well spell it that way. There are some exceptions like don’t add an ‘s’ to Biblical names, but it’s not the kind of thing that comes up much in the kind of writing we do.

Let me just mention the current rule for making a plural word possessive: Just add an apostrophe to the plural form that ends in an ‘s.’

 the doctors’ jackets, the books’ pages, the lions’ manes

If the plural form does not end with an ‘s,’  add an ‘apostrophe s.’

 the children’s hour, the women’s choice, the men’s position

 That’s about it. Easy peasy.

You remember when I said, ‘for most of the world’ at the beginning of this column? This is what I meant by ‘most.’  While most of the world follows these conventions, I have to note that the American Medical Association’s Manual of Style does not—they still use the old way of  only adding an apostrophe if the word ends in ‘s’ as in ‘Jones’.’ That’s unfortunate, but if JAMA editors edit your article this way, please don’t make a fuss. It’s more important to be published. Especially in JAMA. And we know, in our hearts, what’s right.

Tip 2: Accuracy and precision

A reader writes:

That gives me another idea: namely, the difference between the meanings of accurate and precise.

You mean there’s a difference? Only kidding. Of course I know there’s a difference. At least there is in some fields like science, engineering, and statistics. In many dictionaries, accuracy appears as a definition for precision, and precision is listed as a definition for accuracy. While in some fields such as humanities, the two words are viewed as synonymous, in other fields such as statistics, each word has a very specific meaning, and the two are not synonymous at all.

We’re getting into dangerous country here with me talking about statistics. Here goes:

In science and statistics, the words are associated with measurement.  We use accuracy and precision to tell us how confident we should be about the result of the measurement.

Accuracy refers to how close a measured value is to its true (actual) value.

For example, if we measure a one pound bag of flour by putting it on a scale, and it measures 16 ounces or one pound, we can have some confidence that our measurement is accurate.

On the other hand, if we weigh a one pound bag of flour, and the scale reads ten ounces, then, we know the measurement is inaccurate.

Precision refers to how close the results of repeated actions are to one another.

For instance, if we weigh the bag of flour on the scale three times, and one time it comes up as two pounds, one time it comes up as fifteen ounces, and one time it comes up as eight ounces, our measurement is not at all precise. And it doesn’t matter right now whether the fault is in the scale, or we are very sloppy with the flour (that is for a later examination of the reason for the imprecision), we just know the result is imprecise.

On the other hand, if we weigh the bag of flour three times, and each time the weight is the same—regardless of what the weight is—we can be confident that the measurement is precise.

Here is where it gets tricky—here? It has been tricky all along I think.

Something can be accurate but not precise. Using our analogy, the first time we weigh the one pound bag of flour, it weighs 16 ounces, so we are confident it is accurate. We weigh it  more times, and the weights are highly variable, but they average to sixteen ounces (the true weight), so we consider the measurement accurate because the flour weighs 16 ounces on average, but because we got all those varying weights, imprecise. Accurate but not precise.

Something can also be inaccurate and not precise. We weigh the one pound bag of flour and it comes up as twelve ounces, so the measurement is inaccurate. Weighing it more times, we see that the  weights are all very different, and they don’t average to 16 ounces, so the measurement is imprecise. It’s both inaccurate and imprecise.

Something can also be inaccurate but precise. We weigh the one pound bag of flour, and every time we weigh it, it comes up as twelve ounces even though we know it is a pound. It is definitely not accurate, but the repeated measures are clearly precise. Inaccurate but precise.

Finally, something can be accurate and precise. Say we weigh our one pound bag of flour five times, and each time it comes up as sixteen ounces. That is both accurate and precise. Now we are confident that our measurement is accurate and precise.

Now can I move on to something else to make my head stop hurting?


  1. Frankie said,

    Two posts for the price of one this week. Thank you!

    Regarding Tip 1 of this post, can I just add that the exception to omitting the s after the apostrophe doesn’t just apply to biblical names. My dictionary (Oxford) mentions Bridges and Connors.

  2. Thomas L Smith said,

    please add me to your email list

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