10 Common Mistakes Found on Graphs
1. No title on graph
2. Source of data not given
3. Pictograph - no key
4. Scales are interrupted
5. Scales are not labelled
6. Symbols in pictographs not same size or equally spaced on graph
7. Use of 2 or 3 dimensional objects to compare data (area/volume)
8. Scales do not start at zero
9. Numbers on axis (vertical or horizontal) not equally spaced
10. Scale is selected to produce desired result
Four Children
Thirdly we can demonstrate the assessment of probability on this particular example. Imagine parents who would like to have four children. What is the most probable distribution of boys and girls? Do you think that two boys and two girls is the most probable outcome? Unfortunately it is not the correct answer.
If we disregard some biological factors, we can assume that both boys and girls can be born with the same probability 0.5. Note: In fact there is 0.515 probability that a boy would be born, since the long-term statistics have shown that there is about 515 boys out of 1000 children born in average; this is how the nature works, on the other side a man's life is shorter. Let us write out all possible combinations of the children – from boys (B) only to girls only (G).
1. BBBB 2. BBBG 3. BBGB 4. BGBB
5. GBBB 6. BBGG 7. BGBG 8. BGGB
9. GBBG 10. GBGB 11. GGBB 12. GBBB
13. GBGG 14. GGBG 15. GGGB 16. GGGG
We can see that there are 16 combinations or distributions that can occur. As an illustration we can calculate all probabilities.
The probability that only boys would be born is 1/16 (only 1 case out of 16). We can also demonstrate the combinatorial rule of sum – for example in this case: what is the probability that either boys only or girls only would be born? There is the only one possibility that four boys would be born and as well as the only possibility that four girls would be born. The probability that the children would be of the same gender is then 2/16 = 1/8 = 0.125 (or 12.5% if you like).
Now let us go back to our initial question. The probability that two boys and two girls would be born (distribution 6.–11., see the illustration above) is then 6/16 = 0.375.
There are 8 possibilities left, which mean that either 1 girl or 1 boy would be born and the rest of the children would be of a different gender. Therefore this distribution with the probability8/16 = 0.5 > 0.375 is the most probable outcome. Thus the answer, which seemed to be the most likely "at first sight", is not correct.