Q&A #326

Teaching mean, median, and mode (8th grade and below)

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From: Paul Agranoff (for Teacher2Teacher Service)
Date: Mar 13, 1998 at 13:54:09
Subject: Re: Teaching mean, median, and mode

Dear Caron,

I would like to share a lesson I teach to my students about the concepts of
mean, median, and mode.

I believe that students must first understand the concepts of data, and data
handling, before applying them. I begin with a hands-on exploration of data
without numbers. I pull out a board about .5 meter in length, and a handful
of marbles (assorted sizes and colors). With saying a word I let the students
figure out the connection between the board and the marbles. They determine
my key question: how far will a marble roll off an inclined plane? (note:
this is also a great physical science lesson). Students predict what they
think will happen, and then we roll the marbles.

Imagine looking down at the floor of my classroom. The group of marbles (I
start with seven marbles) are now all rolled out from the board. I tell my
students that this is really what data look like. I now have seven different
answers to our one question. Our task is to take seven answers and use them
to answer the key question.

RANGE: I have a student put one of his or her hands in front of the marble
that rolled the farthest, and the other hand behind the marble that rolled
the shortest distance. This student then stands up and shows the class the
range (the difference between the greatest and least data points).

MEDIAN: Marbles do something special that numbers do not. They put themselves
in order from greatest to least. This allow us to find the median (or middle)
data point. I ask students to indicate which marble is the median. They
easily can see that the median has three data points that have a greater
distance, and three data points that have less distance than the median.
(NOTE: they also see that the median is not exactly halfway between the
greatest and least data points. Medians are the middle data point, not in the
middle of the range). It is a great inquiry for students to determine how to
find the median if another marble were added (and even number of data

MODE: Are there some marbles that have rolled the same (or nearly the same)
distance from the inclined plane? If there is some distance that was rolled
more often than any other distance, this is the mode. (NOTE: you won't always
get a mode.)

MEAN: Did all seven marbles roll a fair share of distance from the inclined
plane? Obviously, they didn't. A mean average takes the seven marbles and
gives them each a fair share of distance. I use the "Robin Hood" method of
determining a mean. Robin Hood was famous for "stealing from the rich, and
giving to the poor". We then determine which marble is the richest (greatest
distance) and poorest (least distance). Carefully, we move them towards the
middle at the same rate. Keep doing this with all seven marbles until all
marbles are the same distance from the inclined plane. The process of finding
a mean is taking unfair shares and making fair shares.

I then give each group of students a board and seven marbles. Their task is
to use their marbles to re-answer our key question. When they have practiced
it, and can explain their solution, they call me over to demonstrate what
they can do with central tendency.

Extension: at what angle must the inclined plane be set at to get the maximum
distance of roll on the floor?

I hope this makes sense. My student's really enjoy this, and the concept of
data makes sense to them.

Good luck,

Paul Agranoff

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