## Posts tagged ‘average’

### Why Joe Doesn’t Think He’s Average

Today is an average day, with 182 days left in the year, and 182 days in the rear-view mirror.

I know a lot of average jokes:

When my stats teacher said that I was average, she was just being mean.

With my head in a fire
And my feet on some ice,
I’d say that, on average,
I feel rather nice.

Two men are sitting in a bar when Mark Zuckerberg walks in. One of the men says to his friend, “How awesome! On average, everyone in this bar is a billionaire!”

The last joke highlights the issue with using the arithmetic mean when the distribution would be more meaningful. Let’s assume that one person in the bar has a net worth less than $10,000, two people have net worth between$10,000 and $100,000, and nine people have net worth between$100,000 and $1,000,000. (These are reasonable estimates for the distribution of net worth in the U.S., by the way.) Then a hypothetical histogram with a logarithmic scale showing the net worth of all the people in the bar might look something like this: The average net worth of all 13 people in the bar is over$1,000,000,000 — actually, it’s over $4,000,000,000, because Zuckerberg’s net worth is around$60 billion — but only one of them actually has that much money.

In general, describing data with its average is a terrible idea…

If you’re an “average” person, then you’re a 5’9” (10%) male (50%) with brown eyes (55%) and straight (55%), black hair (85%) who wears size 10.5 (US) shoes (20%). You have 25 teeth in your mouth (30%) — including 4 wisdom teeth (80%) — normal color vision (95%), and O+ blood (40%), but you don’t have dimples (75%). You don’t have hitchhiker’s thumb (75%) or a bent little finger (95%), either, but you can roll your tongue (80%). You have an innie belly button (90%), loops in your fingerprints (65%) instead of whorls or arches, and attached earlobes (65%), and when you interlace your fingers, your left thumb rests atop your right hand (55%). You sleep 6.5 hours per night (15%), smoke 800 cigarettes per year (10%), and consume 2 alcoholic drinks per week (30%). You’re 29 years old (2%), eat 3 servings of fruits and vegetables a day (20%), get 70 minutes of cardio exercise per week (25%), and have a body mass index (BMI) of 25 (15%).

The numbers in parentheses represent the percent of the world population that has the given characteristic. Admittedly, they’re WAGs; I grabbed each statistic from a random location on the web, and I have absolutely no data to back up any of these claims. Moreover, they’re not very precise; I rounded each to the nearest five percent, because a greater level of precision might give the appearance that they’re somehow more accurate.

That said, I don’t think they’re horribly wrong, either, and even if they’re slightly off, they’ll still serve my point. Which is this: Though this description captures an “average” person, it’s pretty far from representing a typical person. The probability that such a person actually exists is only about 1 in 3,500,000,000.

So if you read the description above and thought, “Hey, that’s me!” then you should feel pretty special, indeed — there is likely only one other person in the world with those same characteristics.

The characteristics for the average person used above are sometimes the mean for the category (height, shoe size) and sometimes the mode (eye color, fingerprints). Both of these measures of central tendency are known as averages, as is the median (which I used at the start of this post when claiming that today is an average day).

Life expectancy is another one of those situations where the average provides misleading — or, at least incomplete — information.

Today, the infant mortality rate worldwide is just under 5%, and life expectancy is 71 years. A very simplistic model for this data is to assume that 19 out of 20 people will live to age 75, but 1 out of 20 will die during their first year of life. This model is clearly wrong, but as George Box said, “All models are wrong, but some are useful.” Check out the math with this model:

$\frac{19 \times 75 + 1 \times 0}{20} \approx 71$

This model is useful, because it shows that the 95% of people who survive infancy can expect to live to age 75.

Now compare that to the middle ages, when the infant mortality rate was a staggering 30%, and life expectancy was 35 years. Again using a simplistic model, 7 out of 10 people would live to age 50, while 3 out of 10 would die before they reached the age of 1. The math looks like this:

$\frac{7 \times 50 + 3 \times 0}{10} \approx 35$

So, there’s a problem with using the average to talk about life expectancy, because the distribution in the middle ages was badly skewed by so many childhood deaths.

If we compare life expectancy now to the middle ages using the average of the entire population, it’s a distorted picture. But when we remove the deaths as a result of infant mortality, it’s a little less bleak: those living past age 1 today have a life expectancy of 75 years; those living past age 1 in the middle ages had a life expectancy of 50 years. The scales are still tipped heavily in our favor, but it doesn’t seem quite as drastic as a ratio of 71 to 35.

To put this in perspective, the life expectancy in 1950 was just under 50 years. Most of the increase in life expectancy has actually happened in the last century; during the last 70 years, longevity has increased by more than 20 years.

How typical are you? How long will you live? I have no idea, but I do know this: Half of the people you know are below average.

### A Father’s Day Gift Worth Waiting For

Alex made a Father’s Day Book for me. Because the book didn’t make it on our trip to France, however, I didn’t receive it until this past weekend. It was worth the wait.

The book was laudatory in praising my handling of routine fatherly duties:

I loved when you took me to Smashburger.

I appreciated when you helped me find a worm.

I love when you read to me at night.

I love when I see you at the sign-out sheet [at after-school care]. It means I can spend time with you.

But my favorite accolade — surprise! — was mathematical:

I liked the multiplication trick you taught me. Take two numbers, find the middle [average], square it. Find the difference [from one number to the average], square it, subtract it. (BOOM! Done!)

Priceless.

The trick that I taught him was how to use the difference of squares to quickly find a product. For instance, if you want to multiply 23 × 17, then…

• The average of 23 an 17 is 20, and 202 = 400.
• The difference between 23 and 20 is 3, and 32 = 9.
• Subtract 400 – 9 = 391.
• So, 23 × 17 = 391.
• BOOM! Done!

This works because

$(a + b)(a - b) = a^2 - b^2$,

and if you let a = 20 and b = 3, then you have

$23 \times 17 = (20 + 3)(20 - 3) = 20^2 - 3^2$.

In particular, I suggested this method if (1) the numbers are relatively small and (2) either both are odd or both are even. I would not recommend this method for finding the product 6,433 × 58:

• The average is 3,245.5, and (3,245.5)2 = 10,533,270.25.
• The difference between 6,433 and 3,245.5 is 3,187.5, and (3,187.5)2 = 10,160,156.25.
• Subtract 10,533,270.25 – 10,160,156.25 = 373,114.
• So, 6,433 × 58 = 373,114.

Sure, it works, but that problem screams for a calculator. The trick only has utility when the numbers are small and nice enough that finding the square of the average and difference is reasonable.

Then again, it’s not atypical for sons to do unreasonable things…

Son: Would you do my homework?

Dad: Sorry, son, it wouldn’t be right.

Son: That’s okay. Can you give it a try, anyway?

I’m just glad that my sons understand math at an abstract level…

A young boy asks his mother for some help with math. “There are four ducks on a pond. Two more ducks join them on the pond. How many ducks are there?”

The mother is surprised. She asks, “You don’t know what 4 + 2 is?”

“Sure, I do,” says the boy. “It’s 6. But what does that have to do with ducks?”

### Math for Figger Filbert*

A well-known problem:

A man walks 1 mile south, 1 mile east, and 1 mile north. He arrives at the same place where he started, and then he sees a bear. What color is the bear?

The answer, of course, is white. It’s a polar bear. These three moves will let a person return to the same place if he starts at the North Pole. (The person could also return to the same place if he starts at an infinite number of points near the South Pole, too. He could start at a point so that when he walks 1 mile south, he is at a point such that the east-west circle on which he is standing has a circumference of 1 mile. Then, he can walk 1 mile east to return to the same spot. Finally, he can walk 1 mile north, and he’s back where he started. Then again, he could also start at a point so that he can walk 1 mile south to a point where the circumference of the east-west circle is 1/2 mile, do that loop twice, then walk 1 mile north. Or find points where the cicumference is 1/3 mile, 1/4 mile, 1/5 mile, etc. You get the idea. However, since there are no bears in Antarctica, the answer to my original question is still correct.)

1. In answer to the question, “Are there polar bears in Antarctica?” there is only one correct answer: Only if they are bipolar.
2. I really don’t care to receive silly comments about how a bear trapper could capture a grizzly and take him to Antarctica, or how a brown bear might mistakenly meander north to the Arctic Circle.

Here is a similar question:

A man runs 90 feet, turns left, runs another 90 feet, turns left, runs another 90 feet, and turns left. He is now headed home, and two men with masks are waiting for him. Who are they?

If you don’t know the answer to this riddle, remember that today is the first day of the World Series. My prediction? The Rangers will win easily. It’s not really a fair fight. I mean, members of the Lone Star State’s law enforcement agency with opposable thumbs and automatic weaponry versus defenseless birds? Seriously, if the Rangers don’t win, then we need to seriously reconsider the theory of natural selection.

If you watch the first game of the World Series tonight, remember to enjoy the game. Please don’t get caught up trying to figure out if it converges or diverges.

Here are a few baseball-related math puzzles:

1. A baseball player has four at-bats in a game. At three different times during the game, his batting averages for the entire season (rounded to three decimal places) have no digits in common. What was his average at the end of the game?
2. During a little league game, the visiting team scored 1 run per inning, and the home team scored 2 runs per inning. What is the final score of this seven-inning game?
3. During the first half of the season, Derek batted .100, but his average was .300 during the second half of the season. Similarly, Alex batted .200 the first half of the season and .400 the second half of the season. Both players ended up with the same number of total at-bats, yet Derek had a higher batting average for the entire season. How is this possible?

* Figger Filbert is a term for baseball fans who are obsessed with statistics. Such fans are easily identified; they will make statements like, “Did you know that Albert Pujols is batting .275 when facing married pitchers in suburban ballparks that only sell popcorn on the mezzanine level?” It is a synonym for number nut.

### Stuck in the Middle With You — July 2

Today is an average day, exactly halfway between the beginning and end of the year.

Benoit Mandelbrot, the father of fractal geometry, often said he was born in Poland and educated in France — making him German, on average.

Average is something upon which hens lay their eggs. For instance, “My hens lay four eggs a week on average.”

When she told me that I was only average, she was just being mean.

A statistician with his head in the freezer and his feet in the oven will say that, on average, he feels fine.

Three statisticians go hunting. When they see a duck flying overhead, two of them take a shot. The first fires six inches over the duck; the second fires six inches under the duck; and, the third excitedly exclaims, “We got it! We got it!”

### Analyzing My Favorite Game

I’ve discussed my favorite game before, which is played as follows:

• On a piece of paper, everyone playing writes down a positive integer.
• Show your number to a neighbor (for verification purposes only).
• The winner is the person who wrote down the smallest integer not written by anyone else.

I recently used this game with a group of 32 people at the end of a presentation. The first round was a sample round only, and folks didn’t know the rule for determining a winner before choosing their number. (People often find the rule confusing, so I often do a first round where I don’t tell folks the rule until after everyone has written down a number. Then I give the rule, and we determine the winner to provide an example of the rules in action.)

We played six rounds. I gave the winner of the third and sixth rounds a copy of Math Jokes 4 Mathy Folks; the other rounds were just for fun.

I’ve always been curious about the strategy that folks use when playing this game, so I asked folks to record their numbers for each round, and then I collected their choices. Geek that I am, I analyzed the results, and I thought I’d share them with you. (Don’t you feel special?)

In Round 1, choices were all over the charts. This is to be expected, since folks had no idea why they were choosing a random number. Choices ranged from 3 to 99, with a mean of 15.9.

After the rules were revealed, though, things got more interesting. The charts below show the choices during Rounds 2‑6. (Horizontal axis is the number chosen; vertical axis is the number of attendees who chose the number.)

• The maximum number chosen by any player decreased in each round.
• The average value chosen for Rounds 2‑6 was 8.5, 7.5, 5.3, 7.7, and 6.4, respectively. It’s interesting that the average decreased to 5.3, then shot back up to 7.7. This might be explained by the trend in winning values. The winner in Rounds 2 and 3 chose 2. In Round 4, at least three players chose each of the numbers 1‑4, and the winner chose 5. Players may have assumed that too many players were tending to choose low numbers, so they chose slightly higher numbers in Round 5.
• No fewer than five players chose the number 7 in every round.
• Interestingly, the winner who chose 1 in Round 5 was also the winner who chose 1 in Round 6. She went to the well twice — and it paid off!

A few days later, I ran the same experiment with a different group of 35 people. The results were slightly different.

• The average value chosen decreased in every round, as follows: Round 1, 11.96; Round 2, 7.54; Round 3, 7.53; Round 3, 4.88; Round 4, 4.84; and Round 6, 4.70. As with the previous group, the average took a big dip from Round 3 to Round 4. However, unlike the previous group, the average did not shoot back up in Round 5.
• No fewer than four players chose the number 4 in every round, and it was chosen by 10 players in two different rounds.
• Even after learning the rules by playing a practice round in Round 1, several folks chose surprising numbers in Round 2. Among them: 100; 102; 1,000; 1,900; and 10100.

I hope you enjoyed this diversion. We now return you to our regularly scheduled programming…

### One-Liners

I asked God for a good grade in math class, but I know God doesn’t work that way. So I cheated on my test and asked for forgiveness.

If you get depressed when you think about how dumb the average person is… then you’re probably horrified to realize that half the population is even dumber.

Light travels faster than sound, which is why some people appear bright until you hear them speak.

People who take a long time computing the ratio of rise to run are slope pokes.

Having gone to school doesn’t make you a teacher any more than standing in a garage makes you a car.

To steal ideas from one person is plagiarism. To steal from many is research.

I should’ve known things weren’t going to work out with my ex‑wife. After all, I’m an introverted mathematician, and she’s a lying, cheating, good‑for‑nothing whore.

Mathematicians don’t suffer from insanity. They enjoy every minute of it!

If Bill Gates had a penny for every time I had to reboot my computer — oh wait, he does.

The Math Jokes 4 Mathy Folks blog is an online extension to the book Math Jokes 4 Mathy Folks. The blog contains jokes submitted by readers, new jokes discovered by the author, details about speaking appearances and workshops, and other random bits of information that might be interesting to the strange folks who like math jokes.

## MJ4MF (offline version)

Math Jokes 4 Mathy Folks is available from Amazon, Borders, Barnes & Noble, NCTM, Robert D. Reed Publishers, and other purveyors of exceptional literature.