Internal reliability – Cronbach alpha

2.3.1. Background

Having decided to retain three components from the PCA based on a logical assessment of their respective items, a further statistical analysis – in the form of Cronbach α - was performed using SPSS, in order to confirm or challenge these conclusions.

Looking at the output generated in SPSS, the following three tables need to be consulted in order to draw conclusions:

a) “Reliability Statistics”

According to this table, a minimum value of α = 0.7 – 0.8 is required in order for the respective scale to be considered to have a good internal reliability, thus indicating that all questions are measuring the same thing.

b) “Item-Total statistics: Corrected item – total correlations”

These values illustrate the correlations between each item and the total score retrieved from the questionnaire. In order for a scale to be considered reliable, all items must correlate with the total. Otherwise, if any of the items has a value less than 0.3 it might need to be dropped.

c) “Item-Total statistics: Alpha if item is deleted”

These values indicate the overall alpha value if each given question were to be removed from the calculation. Therefore, when reading this table we are looking for values that are greater than the overall alpha, because if the deletion of an item increases Cronbach alpha that means it improves the scale’s reliability.

2.3.2. Internal reliability for each individual sub-scale produced by PCA

2.3.2.1. Subscale 1: Perceived behavioural control

This subscale appeared to have good internal consistency with α = .804. In addition, all items are sufficiently correlated with the total (values greater than 0.6), while none of them would substantially affect reliability if they were removed (see Appendix 8). Therefore, all three items appear to be worthy of retention.

2.3.2.2. Subscale 2: Benefits of amniocentesis / CVS

Similarly this subscale had good internal consistency with α = .738. All items were correlated with the total (values greater than 0.5) and none of them would increase reliability if they were to be removed (see Appendix 8). Therefore, all three items were suitable to be retained.

2.3.2.3. Subscale 3: Attitudes towards chromosomal abnormalities

Contrary to the other two constructs this subscale had a low internal reliability score with α=.466 (see Appendix 8). This may be due to the fact that the specific sub-scale only consisted of two items which may not have been sufficient enough to measure what they set out to measure.

According to Tavakol & Dennick (2011) “if the test length is short, the value of alpha is reduced”.

Therefore, they recommend that in an attempt to increase internal reliability, more items testing the same concept can be added to the test. This is a good point to be explored by future studies.

However, despite the low alpha score both items correlated with the total, meeting the threshold criterion of 0.3 and therefore at least for the purposes of this study this subscale will be retained as it is though results will be interpreted with cautiousness.

2.3.3. Internal reliability for other subscales included in the questionnaire

As previously mentioned, PCA was only performed on those sections of the questionnaire that were developed for the purposes of this study and needed to be assessed for their psychometric properties. However, there were other scales also included in the questionnaire that have previously been validated through other studies. Nevertheless, as it has been suggested to be good practice (Streiner, 2003) it was decided that a reliability test would also be applied to these individually in order to test for their suitability for this particular population (Streiner, 2003) (see Appendix 9).

a) Hospital Anxiety and Depression Scale (HADS) – Anxiety scale

This subscale seemed to have a lower level of internal reliability than expected, with α=.577.

b) Attitude towards doctors and medicine

This subscale also had a low internal reliability based on this sample, with α=.601 c) Health Locus of Control (HLOC)

Finally, this subscale also demonstrated low internal reliability for our sample, with α=.563.

However, despite the low internal reliability scores for all three above scales, they will be retained in the questionnaire for the main study. Several previous studies have validated their psychometric properties and therefore the low alpha scores observed in this study may be more related to the size of the sample but may not be an issue in the main study where the sample will be significantly larger.

It is important to note here that the main purpose of this pilot study was mainly to assess those sections of the questionnaire that were newly developed, through the use of PCA and Cronbach alpha and thus the reliability of the latter three scales will again be assessed in the main study.

2.3.4. Questionnaire amendment based on PCA findings and cronbach alpha All in all, based on the PCA the following factors will form part of the amended version of the questionnaire (see Appendix 10) to be used in the main study together with the previously validated scales mentioned in 2.3.2 above.

A detailed account of all factors included in the original and the amended questionnaire is illustrated in Table 2.7. The amended questionnaire will be referred to as Prenatal Decision Making Questionnaire (PDMQ) for the remainder of the study.

Table 2.7

Factors included in the original and the amended questionnaire