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. 2021 Sep 10;21(18):6088.
doi: 10.3390/s21186088.

The Sample Size Matters: To What Extent the Participant Reduction Affects the Outcomes of a Neuroscientific Research. A Case-Study in Neuromarketing Field

Alessia Vozzi  1   2 , Vincenzo Ronca  1   2 , Pietro Aricò  2   3 , Gianluca Borghini  2   3 , Nicolina Sciaraffa  2   3 , Patrizia Cherubino  2   3 , Arianna Trettel  2 , Fabio Babiloni  2   3   4 , Gianluca Di Flumeri  2   3
Affiliations

The Sample Size Matters: To What Extent the Participant Reduction Affects the Outcomes of a Neuroscientific Research. A Case-Study in Neuromarketing Field

Alessia Vozzi et al. Sensors (Basel). .

Abstract

The sample size is a crucial concern in scientific research and even more in behavioural neurosciences, where besides the best practice it is not always possible to reach large experimental samples. In this study we investigated how the outcomes of research change in response to sample size reduction. Three indices computed during a task involving the observations of four videos were considered in the analysis, two related to the brain electroencephalographic (EEG) activity and one to autonomic physiological measures, i.e., heart rate and skin conductance. The modifications of these indices were investigated considering five subgroups of sample size (32, 28, 24, 20, 16), each subgroup consisting of 630 different combinations made by bootstrapping n (n = sample size) out of 36 subjects, with respect to the total population (i.e., 36 subjects). The correlation analysis, the mean squared error (MSE), and the standard deviation (STD) of the indexes were studied at the participant reduction and three factors of influence were considered in the analysis: the type of index, the task, and its duration (time length). The findings showed a significant decrease of the correlation associated to the participant reduction as well as a significant increase of MSE and STD (p < 0.05). A threshold of subjects for which the outcomes remained significant and comparable was pointed out. The effects were to some extents sensitive to all the investigated variables, but the main effect was due to the task length. Therefore, the minimum threshold of subjects for which the outcomes were comparable increased at the reduction of the spot duration.

Keywords: EEG; GSR; HR; applied neurosciences; sample size; signal processing.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
The graph represents the trend of ‘v’, the mean value of Index1 computed over the entire population (36 subjects) during SPOT1 (ALL). The dashed lines show the trend of the maximum and minimum values (Max, Min) of the index among the ‘v630’ for each second, in each subgroup of subjects (32, 28, 24, 20, 16).
Figure 2
Figure 2
The box plots represent the effect of the subgroup of subjects on (a) Rho values, (b) MSE values, and (c) STD values, computed for Index 1, during the SPOT 1.
Figure 2
Figure 2
The box plots represent the effect of the subgroup of subjects on (a) Rho values, (b) MSE values, and (c) STD values, computed for Index 1, during the SPOT 1.
Figure 3
Figure 3
Graphical representation of the trend of rho correlation coefficient at reducing the sample size (subgroups) with respect to the ‘full population index’ (36 participants). Tasks (a) and (b) were 30 s long, Task (c) 20 s, and Task (d) 15 s long.
Figure 3
Figure 3
Graphical representation of the trend of rho correlation coefficient at reducing the sample size (subgroups) with respect to the ‘full population index’ (36 participants). Tasks (a) and (b) were 30 s long, Task (c) 20 s, and Task (d) 15 s long.
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