Anna and Troy’s Weblog

Amy’s comment on my last post sparked my curiosity regarding accuracy vs. precision….one of the many many many things that I once knew in college and have since forgotten……there are an awful lot of those things. I actually think that I have forgotten more things than I remember. I can’t decide if that is a sad commentary on myself, on the educational system, or just a cosmic truth that should be accepted without too much worry…. Food for thought.

For those of you that are interested, see below (it might just be you and me, Amy).

ACCURACY AND PRECISION

ACCURACY

Accuracy describes the nearness of a measurement to the standard or true value, i.e., a highly accurate measuring device will provide measurements very close to the standard, true or known values.Example: in target shooting a high score indicates the nearness to the bull’s eye and is a measure of the shooter’s accuracy. Refer to pictures below:

PRECISION

Precision is the degree to which several measurements provide answers very close to each other. It is an indicator of the scatter in the data.The lesser the scatter, higher the precision. The pictures given below clearly describe Accuracy and Precision.

EXAMPLES

If we measure the length of a foot-ruler and get values of 12.01 in, 12.00 in, 11.99 in, 12.00 in. These numbers are precise enough for us to believe that if we measure it again we would get 12.00+0.01 in. These measurements are precise but necessarily accurate. The foot-ruler may actually be metric ruler of 30.0 cm long. Our measurement is precise but not accurate.

APPROXIMATIONS

Even though physicists usually try for a high degree of precision, there are times when only a close approximation is needed. Physicists some times make rough estimates for making tentative decisions. The accuracy of estimates depends on reference materials available, time devoted, and experience with similar problems.

ORDERS OF MAGNITUDE

Some times physicists make orders of magnitude calculations. In these calculations, the numbers may differ by a factor of ten. Example:106 m 10,980 m differ by two orders of magnitude.

Here is a cosmically relevant question for y’all:

Why are you always so much heavier at the doctor’s office?

Is this not always true? I have found this to be a cosmic, scientifically replicable truth….much like gravity. In the science experiment that is MY life, this hypothesis has yet to be disproven.

I have to say that I have generally not been a meticulous scale watcher. I am pretty aware of the 5 pound window that is my comfortable “normal” weight. If I am feeling a little overindulged or chunky, I usually feel it first and then my suspicions are confirmed by my scale….better lay off the late night ice cream, okay….done.

Now that I am pregnant, I have been watching the scale with a little more vigilance. A week and a half ago I was horrified to discover, that according to the scale at my OBGYN’s office that I had gained 7.5 lbs! Yikes! (A quick reminder: at that visit we were at about 10.5 weeks….which means that the baby is the size of a large olive, and when they say “large” they don’t mean those famous 7.5 lb. olives that you hear about all the time.)

Now to put this in perspective for those of you who are not spending all of their spare time reading baby books, according to the books, a woman who starts at a “normal healthy” weight, should gain between 30 and 35 lbs. over her ENTIRE pregnancy. The majority of this weight is generally put on in the second and third trimesters (when the baby is considerably bigger than an olive). So, 7.5 pounds is not a completely ludicrous amount of weight gain for the first trimester, but it is on the high end of the first trimester scale.

Now that I have had a little time to adjust, I am feeling better, and upon further analysis, my initial shock and horror sprang from two major sources.

One (this is the one that I have reconciled): I have to assume that every first-time-pregnant-woman does a little bit of a double take as she first sees the numbers on the scale go up in a significant way. My little “growth spurt” pushed me over my own “emergency alert weight”. Every woman, perhaps every person, has one of these. It is the number that we silently keep in our head that means, “This is an unacceptable amount of weight gain, and it definitely has its root is too many cookies and frappuccinos….. No longer can this be blamed on bloating, water retention, or my unfortunate wardrobe choice of the-heaviest-clothes-that-I-own.” Now, being a relatively sane person, I have come to realize that pregnancy is going to require a re-evaluation of my “emergency alert weight,” and after some self reflection I feel that I am ready to accept this.

Two (this is the one that is sort of boggling my mind): My scale at home did not warn me ahead of time. In fact, according to my home scale, I had only gained a couple of pounds. So of course, one of the first things that I did after recovering from the initial shock of my doctor’s visit, was to go out and buy a new and much higher-tech scale….a scale that promised to be accurate within a half of a pound up to 330 pounds…….and SURPRISE…..it too registered a lower weight than the evil scale at the doctor’s office (although, slightly higher than the old discarded scale, but not over my “emergency alert weight.”)

So how do we reconcile this phenomenon? Someone has to be wrong. I have always assumed (although with no real grounds) that the doctor’s scale would win for accuracy….and I suppose in the end it doesn’t really matter, but it is an interesting phenomenon nonetheless.

As for my current situation, I suppose that the actual number on the scale isn’t as important a measure as the relative change in said number. So I guess I am okay with that…..and I suppose that I can accept the fact that I will always weigh 3-5 lbs. more on the doctor’s scale than I will on my own.

p.s. According to my new high-tech home scale, I am still under my “emergency alert weight;” although, just barely. Talk to me again tomorrow and it may be a different story.