Figuring out the population figures

 By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

05 September 2010

After an 11-month wait, the census results were released this week and it turns out that there were about 38.6 million people living in Kenya in September last year. This is lower than the 40 million figure that has been thrown around in recent months.

Compared to the 1999 result, the population has increased by about 10 million people in the 10-year period. Thus, many have concluded that we have been growing at the rate of one million per year. Is that correct? Let’s find out.

The population recorded during the 1999 census was 28.7 million; the current figure is 38.6 million. Therefore, in the ten-year period, the number of people has increased by about 34.5 per cent. Does this mean that we have grown by an average of 3.45 per cent since 1999; that is, 34.5 divided by ten?

No! That is not the right way to calculate average percentage rates. If you doubt me, start with 28.7 and add 3.45 per cent ten times over and see. That is 28.7 plus 3.45 per cent, equals 29.69; 29.69 plus 3.45 per cent equals 30.71; and so on. The final answer is over 40 million.

To get the correct average annual growth rate, we start by finding the ratio of the current to the 1999 population. That is; 38.6 divided by 28.7. I have dropped the million because it cancels out in the division. The answer is 1.345.

Next, we ask ourselves: What number would be multiplied by itself 10 times and produce 1.345? In other words, what is the tenth-root of 1.345? The answer is 1.030. If we start from 28.7 million and keep multiplying by 1.030 ten times, we shall get 38.6 million. Try it and see for yourself.

Now, multiplying a number by 1.030 is equivalent to adding 3 per cent to it. Therefore, in the last ten years, the population of Kenya has been growing at an average of 3 per cent per year.

So; from 1999 to 2000, the number of Kenyans increased by approximately 861,000 (3 per cent of 28.7 million) to make a total of 29.561 million people. The following year (2001) the number jumped by 887,000 (3 per cent of 29.561 million) to 30.445 million, and so on.

The year that recorded an increase of one million people is probably 2006; and if the past trend continues, we shall have an additional 1.15 million people by December this year bringing the population very close to 40 million – 39.7 million.

Now 3 per cent growth per year might sound like a slow rate but, if this continues, the 2019 census will record 52 million and by 2029 we shall be almost 70 million people. It is now wonder that planners are scratching their heads – how shall we feed all these people, for example?

Finally, results from some districts were cancelled because of anomalies in the data. I shall attempt to explain the problem in a future article.