Addressing potential limitations

All research studies involving human participants have limitations due to the unpredictability and variability of human behaviour (Cziko, 1989, Willerman, 1979). This means that while a plan is essential, it is equally essential to be flexible enough to modify the study approach slightly in response to participant characteristics and needs.

PhD studies are constrained by limited human, financial and temporal resources. Although a longitudinal design would have offered a clearer interpretation of causal relations, there is insufficient time within a PhD to conduct a study with a follow-up period longer than six months. The relative benefit of such a short period of follow-up over and above a cross-sectional design is highly questionable. Moreover, due to attrition bias, longitudinal decisions require a larger initial sample size to produce the same sample size at follow-up that would be achieved using a cross-sectional design. Therefore, a cross- sectional design was most appropriate for my study.

Additionally, in PhD studies, a short recruitment period and a lack of financial resources to run multiple research sites or have other research staff, impose restrictions on sample size. This prevented the use of principal components analysis, which could have provided a useful conceptual grouping of acoustic variables prior to regressing against listener outcomes. Moreover, phonetic analysis is highly resource intensive. Since I performed all the phonetic analysis, it was only possible to perform phonetic analysis on

a subset of participants. Additionally, as discussed in section 5.4.4, there were a small number of phonetic measures which I did not have time to complete. Performing phonetic analysis on conversational speech samples is intrinsically considerably more challenging than analysing read sentences, due to non-standard content, which introduces a wide range of potential sources of variance.

However, sample size is a frequent challenge for studies of speech and communication, not solely those conducted in the context of a PhD. Since phonetic analysis is resource intensive in terms of parameter refinement and conduct of analyses, sample sizes are often restricted, unless a large number of skilled analysts are available. As indication of the magnitude of the challenges faced, phonetic studies cited in chapter two of this thesis included on average fewer than fifteen PwPD. Listener studies using standardised read sentences also face sample size limitations, due to the potential of stimulus learning effects (see section 5.5.3.1). In my systematic review of extant

knowledge about relationships between cognitive status, and speech and communicative impairments in PD, included studies recruited a median of twenty PwPD. In the context of studies in other aspects of PD, this is not particularly large.

Limited sample size makes it difficult for studies in this field to have satisfactory statistical power for fine-grained well-controlled statistical analyses. For practical and design reasons outlined earlier in this thesis, I could only include twenty PwPD and twenty CPs in my speech analyses. This means that my sample size for this aspect of my project was above average for the field, but smaller than ideal. Therefore, I adopted a two-tier approach to sample size. For the communicative analyses, in which the above restrictions did not apply, I used a sample size of forty-five PwPD, in order to provide greater statistical power for the intended analyses and to increase generalisability. My sample size for communicative analyses was larger than all communicative studies in my systematic review, with the exception of Miller et al (2008, 2011).

Although this was partly redressed by purposive sampling for speech measures, there was a sampling bias towards PwPD who had mild speech impairment. The

underrepresentation of those with moderate-to-severe speech impairment may relate to increased self-consciousness of people with moderate-to-severe speech impairment about being recorded. Anecdotal evidence for this suggestion comes from the receipt of several responses to study invitations, saying that while the person was interested in the project and supportive of research, he or she did not feel able to participate in this particular study, as a result of self-consciousness about either the topics of speech and communication or about being recorded.

Although the inclusion criteria for my study (see section 4.4.2) exclude people with dementia, identification of dementia prior to invitation relied on clinic records. Since PwPD were invited by clinic staff two weeks prior to attending clinic, it is possible that some may not have been assessed for a while. Therefore, it is possible that some people who had no diagnosis of dementia could have declined cognitively since their previous

appointment, to an extent where they may have had mild dementia at the point of invitation. However, my study investigated the impact of a wide range of cognitive status on speech and communicative outcomes, rather than the putative concept of mild cognitive impairment as strictly defined. On no occasion did I visit a potential participant and subsequently find that he or she was incapable to consent.

Any misunderstandings about the nature of the study were minor and resulted from lack of prior experience of the topic or research, rather than dementia. Where misunderstandings arose, I provided clarification before seeking consent. On one

occasion, following telephone discussion between the carer and me, it was decided that a study appointment should not be made for one interested potential participant, due to concerns by the carer and a consultant physician about dementia. This situation arose because invitations were sent out two weeks before the patient’s next clinical

appointment. This means that if the PwPD or a relative had any significant concerns regarding dementia, they could discuss them with the clinical care team prior to deciding whether to participate in my research study. Therefore, any dementia amongst the included participants would have been mild.

MoCA scores provide an indication of how many participants in my study may have had mild dementia. Nasreddine et al (2005) report MoCA scores between 11 and 21 for people with Alzheimer’s disease, with a mean of 16 and standard deviation of five. Fifteen of my participants had a MoCA score of ≤21. However, only three had a MoCA score ≤16. Therefore, it is probable that a small number of my participants had mild dementia. Due to the nature of the investigation, the impact of this on the interpretation of my results is low. My inclusion criteria did not exclude intact cognitive status, since I wanted to be able to compare the impact of a range of cognitive status. Nine of my participants showed evidence of intact cognitive status according to MoCA. MoCA score was not available for one of my 45 participants. Therefore, with a small number of exceptions, it appears that my study recruited its intended sample.

Due to the ethical requirement to use university members as assessors, listeners were not demographically similar to speakers in intelligibility and emotional conveyance assessment. University members are younger and more highly educated than the general population. In addition, since my study only had the funds to offer a modest prize draw,

most assessors came from schools of study that were to some extent related to the topic of my study. Since the majority of students in these schools are female, and men are known to be harder to recruit into studies, 88% of the assessors in my study were female. This contrasts with 35% of PwPD and 65% of CPs in the speech analysis sample.

However, older people converse with younger people, men with women and the more educated with the less educated. Therefore, it is unclear whether these demographic differences between speakers and listeners would have had any impact on the results of listener assessment.

It would have been interesting to have included an age-matched assessor group. However, this would have involved recruiting assessors from the community. I was advised that, for ethical reasons, university members should be used as assessors since pre-existing contractual arrangements provide additional safeguards should assessors recognise any speakers. The only way it could have been possible to use an age-

matched community-based assessor sample would have been to recruit assessors from a different region of England. This would have posed considerable challenges in terms of advertising, finding suitable venues to conduct the assessment sessions, cost and appropriate transport of personal data.

Although the conversations included in my study can be called semi-naturalistic, it is not possible to obtain fully natural speech from people who know they are being

observed. This is called the Observer’s Paradox (Cukor-Avila, 2000, Labov, 2006, Labov, 1966) and is a challenge for all social psychological and sociolinguistic investigations. Under current UK law and ethical standards, all studies seeking to obtain naturalistic behaviour will face this limitation. To partly mitigate this limitation, any conversations that appeared significantly unnatural were excluded from analysis by purposive sampling (see section 5.4.2).

MoCA, HADS, CES and CPIB were not measured in the CP group. CPIB is not suitable for comparing the communicative participation of a patient group with a control group because it asks how much a person’s communicative participation has changed since having a condition. It was decided that administering MoCA, HADS and CES to CPs would make the data collection session excessively long. The absence of these baseline measures for CPs meant that I could not control for HADS or MoCA in group comparisons of speech acoustics. Additionally, intact cognitive status and communicative participation in CPs had to be inferred. The existence of impaired cognitive status and communicative participation in PwPD in my study had to be inferred from normative data and extant knowledge, rather than through group comparison within my sample. With

regard to communicative participation, QCA communication results addressed this limitation.