Maths misconceptions
Everyone knows the feeling of struggling with a task that
other people seem to breeze through. It might be programming the DVD player or
even just reading maps.
Well, this is how some kids feel with maths, and their
difficulties are often rooted in misunderstandings of concepts that we, as
teachers, don't give a second thought to. How much could we help them make
progress if we were more aware of these misconceptions, and were able to tackle
them headon? We all know, after all, that understanding our mistakes can be a
powerful learning experience.
With the help of Tim Coulson, who leads the National
Numeracy Strategy, we've put together a list of some of the most common,
and potentially most obstructive, of these misconceptions, and suggest some
approaches that might put things right.
1. A number with three digits is always bigger
than one with two
Some children will swear blind that 3.24 is bigger than 4.6 because
it's got more digits. Why? Because for the first few years of learning,
they only came across whole numbers, where the 'digits' rule does
work.
2. When you multiply two numbers together, the
answer is always bigger than both the original numbers
Another seductive 'rule' that works for whole numbers, but
falls to pieces when one or both of the numbers is less than one. Remember
that, instead of the word 'times' we can always substitute the word
'of.' So, 1/2 times 1/4 is the same as a half of a quarter. That
immediately demolishes the expectation that the product is going to be bigger
than both original numbers.
3. Which fraction is bigger: 1/3 or 1/6?
How many pupils will say 1/6 because they know that 6 is bigger than
3? This reveals a gap in knowledge about what the bottom number, the
denominator, of a fraction does. It divides the top number, the numerator, of
course. Practical work, such as cutting predivided circles into thirds and
sixths, and comparing the shapes, helps cement understanding of fractions.
4. Common regular shapes aren't recognised for
what they are unless they're upright
Teachers can, inadvertently, feed this misconception if they always
draw a square, rightangled or isosceles triangle in the 'usual'
position. Why not draw them occasionally upside down, facing a different
direction, or just tilted over, to force pupils to look at the essential
properties? And, by the way, in maths, there's no such thing as a diamond!
It's either a square or a rhombus.
5. The diagonal of a square is the same length as
the side?
Not true, but tempting for many young minds. So, how about challenging
the class to investigate this by drawing and measuring. Once the top table have
mastered this, why not ask them to estimate the dimensions of a square whose
diagonal is exactly 5cm. Then draw it and see how close their guess was.
6. To multiply by 10, just add a zero
Not always! What about 23.7 x 10, 0.35 x 10, or 2/3 x 10? Try to spot,
and unpick, the 'just add zero' rule wherever it rears its head.
7. Proportion: three red sweets and two blue
Asked what proportion of the sweets is blue, how many kids will say
2/3 rather than 2/5? Why? Because they're comparing blue to red, not blue
to all the sweets. Always stress that proportion is 'part to
whole'.
8. Perimeter and area confuse many kids
A common mistake, when measuring the perimeter of a rectangle, is to
count the squares surrounding the shape, in the same way as counting those
inside for area. Now you can see why some would give the perimeter of a
twobythree rectangle as 14 units rather than 10.
9. Misreading scales
Still identified as a weakness in Key Stage test papers. The most
common misunderstanding is that any interval on a scale must correspond to one
unit. (Think of 30 to 40 split into five intervals.) Frequent handling of
different scales, divided up into twos, fives, 10s, tenths etc. will help to
banish this idea.
Words: Steve McCormack
Pictures: David Moore
For more info
Did you know?
In 2005, 75% of pupils attained level 4 or higher at KS2
maths... More than 110,000 pupils were entered in the recent Primary
Mathematics Challenge (Source Mathematical Association).
