Results:
Total mean for natural fibres = 1372.5g
Total mean for synthetic fibres = 1084g
Analysis:
I have decided that the fourth result for quilting cotton is an outlier and have not included this when calculating the mean strength. I also think that the third result for the second polyester and the second for wool are outliers and have not used them.
I do not think there is any real difference between the strengths of synthetic and natural fibres, although natural fibres do have a higher total mean. Some of the fibres, for example wool and the first polyester, had a strength that was much higher than the other fibres. They also seemed to have a wider range (if you are including the outlier).
Most of the results for the threads are reliable. There are only three outliers but if they are discounted the results for viscose, nylon, the second polyester, polyester/polyamide, cotton, quilting cotton and silk all have very small ranges. This is especially true of viscose and polyester/polyamide.
The data was collected safely and only minimal safety procedures, such as using a lab coat to cushion the fall of the weights, were needed. However, it was difficult to tie all of the knots in the same way and to ensure they were secure. An easier way of attaching the threads to the clamp stands would have helped to make the results more accurate. The clamp stands were also not the correct height, as the fibres stretched, making the height needed larger. Taller clamp stands could have been used to ensure this did not happen.
My hypothesis was incorrect as there was no significant difference between the strengths of the thread. If anything, the natural fibres were stronger (the total mean was 288.5g higher than that for synthetic fibres). This could be because synthetic fibres are made to imitate natural fibres at cheaper cost and therefore not as good.
My conclusion is quite reliable as most of my data is reliable. There is also a smaller probability of unnoticed anomalies being included because each fibre was tested five times. The techniques used were easy to repeat if care and time was taken. I think that because of these factors I can have a high level of confidence in my conclusion being correct.
Evaluation:
The differences in the strengths of the fibres could be caused by human error. It is possible that the fibres were pulled and weakened or that more knots were tied in the fibres than in other tests for the same thread.
There was not enough time to thoroughly investigate the strength of the thread. We were sometimes rushed and could have made mistakes because of our haste. We could also have tested the fibres more and tested a larger number of fibres if we had had more time to conduct the experiment.
We did not check that the weights were all the exact weight that they were marked as being. This means that some of the weights could have been lighter or heavier than perceived and outliers may be due to this.
The outliers could also be because of an actual difference in the strength of the fibre. The results for the first polyester, for example, are very varied but none of them are extremely far from the other results. This could be because the fibre’s strength varied or portions of the fibre could have been weaker.
