Assessment of agreement between a new electronic scale and mechanical suspended scale for measurement of children’s weight in Ethiopia
By Asrat Dibaba, Barbara Main and Mark Myatt
Location: Ethiopia
What we know already:Mechanical spring suspended scales are commonly used for measurement of weight in children under five years of age. Use requires training and experience. Small changes in weight are difficult to measure (accuracy ±100g).
What this article adds:A new electronic scale is available that detects smaller changes in weight (accuracy ±20g), is easier to use, can be used interchangeably with mechanical scales and is of equivalent price.
Asrat Dibaba is the Maternal Child Health and Nutrition Advisor for World Vision East Africa Region based in Nairobi, Kenya. He has worked in Ethiopia, Liberia, Sudan and Cambodia.
Barbara Main is a Public Health Specialist at World Vision Canada. She has extensive experience supporting maternal and child health and nutrition programming in Africa and Asia, including nine years based in Cambodia.
Mark Myatt is a consultant epidemiologist. His areas of expertise include surveillance of communicable diseases, epidemiology of communicable diseases, nutritional epidemiology, spatial epidemiology, and survey design. He is currently based in the UK.
The authors declare that they have no competing interest.
The authors would like to thank World Vision Ethiopia, in particular Dr. Tilaye H/Micahel, Mr. Abiy Worku and health workers at the three health posts in Shone district, for their support during the field work. The authors would also like to thank World Vision Canada for supplying the electronic scales.
Mechanical spring suspended scales (e.g. Salter model 235-6S) are commonly used for measurement of weight in children under five years of age in many developing countries. This type of scale can weigh a child up to 25 kg and is graduated in 0.1 kg (100g) increments. Adequate training and experience is needed to correctly use this type of scale. Even with adequate training and experience, small changes in weight are difficult to measure. A more accurate and easier to use scale is needed to measure small changes in weight. The field test reported here was carried out to test a new electronic scale which may be suited to use in primary care settings and by community health workers at field level.
Method
The mechanical scale was manufactured by Salter Brecknell (model 235-6S PPW Baby Weigher). The electronic scale was manufactured by Rice Lake (model OS-25 Suspended Scale). Table 1 shows a comparison between the two scales.
The field test was conducted in Southern Ethiopia in December 2012. Weight measurements were performed on 60 children aged between 6 and 59 months who came to three health posts for Outpatient Therapeutic Programme (OTP) follow up or to access other medical services. Each child was weighed using both the mechanical and the electronic scales. All measurements were taken by experienced health workers.
| Table 1: Comparison of electronic versus mechanical scales | |||
| Attribute | Electronic Scale | Mechanical Scale | Notes |
| Time for stable weight | 1 - 2 seconds | 2 - 5 seconds | |
| Weight displayed after weight removed | Yes | No | Makes the scale easier for a single person to use |
| Accuracy | ± 20 g | ± 100 g | |
| Calibration | Electronic | Manual | Automatic for the electronic scale |
| Indicator display | Digital | Analogue dial | |
| Weight of scale | 350g | 666 g | |
| Handheld operation | Yes | No | Electronic is easier for survey work |
| Power supply | 2 Ã AA Batteries | Not required | Auto-off feature extends battery life |
| Operating temperature | 10° C - 40°C | 0° C - 40° C | |
| Hooks are secured | Yes | No | Bottom hook fixed in electronic scales. Use the same current accessories. |
| Easier for one person to use | Yes | ||
| Cost | $75 | $95 | Manufacturer list price (lower prices may be negotiated) |
Data were analysed using Bland & Altman plots. This is a semi-graphical method used to compare two measurement techniques in which the difference between pairs of measurements are plotted against the average of the paired measurements in order to assess the agreement between two methods of measurement1.
Results
A Bland & Altman analysis for agreement was done using weight measurements from both scales (Figure 1). The solid horizontal line marks the mean difference (-0.037 kg). The dotted horizontal lines mark the 95% limits of agreement (i.e. the mean difference plus or minus 1.96 times the standard deviation of the mean difference). The mean difference was 0.037 kg. The 95% limits of agreement were -0.129 kg and 0.055 kg. Accordingly, the new electronic scale may weigh 0.129 kg above the mechanical scale or 0.055 kg below the mechanical scale. The absence of a strong pattern in the Bland & Altman plot (Figure 1) indicates that the observed differences are unrelated to the magnitude of measurement.
Conclusion and recommendations
The measurement differences found in this study are not clinically important as the large majority (i.e. 92%) of differences is within the manufacture’s quoted accuracy (i.e. 0.1 kg) for the mechanical scale. This difference is acceptable in nutrition rehabilitation and growth promotion programmes, therefore, the two scales can be used interchangeably for weight measurement in children under five years. For more accurate assessment of small differences between two measurements, the digital scale is recommended for use, especially by less trained health workers at community level.
For more information, contact: Asrat Dibaba, email: asrat_dibaba@wvi.org
1Bland JM, Altman DG (1986). Statistical Method for Assessing Agreement between Two Methods of Clinical Measurement, Lancet 1986, i:307-310
Imported from FEX website