Comparing data over time is never easy. The methodologies and instruments engaged to collect information need to be comparable; seasonally adjusted data must be compared with other data that have also been seasonally adjusted, not with unadjusted data; and quarter-on-quarter data are best compared with data from an equivalent period in a previous year.
This last point is particularly important, because unless quarterly data have been seasonally adjusted, seasonal variations can lead to distortion when comparing a quarter in one year with a different quarter in another year. These basics of statistical practice can be reduced to a simple injunction: always compare like with like.
Comment on Tuesday's release of latest data on quarterly employment statistics (QES) well demonstrates the necessity to compare like with like. In at least one newspaper report, there was an attempt to compare seasonally adjusted figures for gross domestic product with unadjusted figures for employment.
A report also compared jobs created in the fourth quarter of 2005 with jobs created in the first quarter of 2006, without taking account of the variations in economic activity likely to occur in a quarter that includes peak holiday time compared with the first quarter of a new year.
A separate headline in another newspaper quoted Statistics SA as saying that growth in employment was falling behind target. Nothing of the sort had been said.
A more careful look at the latest employment statistics shows that there was an increase in the number of jobs created in the quarter ended March this year when compared with the equivalent quarter ended March last year.
Between January and March 2005, the economy shed 152 000 jobs; in the same quarter this year, 9 000 new jobs were created.
In the quarter ended March 2005, Stats SA recorded 6 945 000 employees, with gross earnings of R146.47 billion. In the equivalent quarter (January-March) of this year, the QES recorded 7 257 000 employees with gross earnings of R164.38 billion. This reflects an increase between quarters of 4.5 percent in respect of jobs, and 12.2 percent in total earnings.
Data can also be compared year on year, provided the criterion for comparison of like with like is adhered to. According to the latest QES survey, 7 257 000 people were employed in the formal non-agricultural business sector of the economy as at March 2006. This reflected an annual increase of about 312 000 people, or 4.5 percent, compared with March 2005, which recorded 6 945 000 employees.
The largest contributor to this increase came from the financial services sector, where the number of jobs rose by 102 000. Other main sectors creating jobs were construction (83 000), trade (60 000) and personal services (49 000).
The gross earnings paid to employees during the three months to the end of March this year amounted to R164.38 billion, reflecting an annual increase of R17.94 billion, or 12.2 percent, compared with the quarter ended March 2005.
Headlines can be misleading, especially when dealing with comparisons over different time periods.
The number of new jobs created when comparing the last quarter of one year with the first quarter of a new year may appear to be a "trickle", as one newspaper has suggested. However, deeper analysis reveals a different picture, with 312 000 new jobs being recorded year on year - rather more than a trickle.