research paper (stats, sway calculations, results) Flashcards
when was the data filtered? how?
filter with a cut-off frequency of 10 Hz done before calculation of the sway characteristics.
how to calculate path length
Path Length was calculated as the sum of the distance between each sequential point of measurement
how to calculate area
Area was calculated as the total enclosed area of the participant’s movement using the minimum enclosed polygonal area of the plotted outer vertices using Matlab’s convex hull function
- taking the area of the ecnlosed shortest distance between all outlying points
how to calculate mean coronal velocity
The Coronal Velocity was calculated as the mean of the absolute Coronal displacement per sampling period.
how to calculate lateral coronal displacement
Lateral Coronal Displacement was calculated as a difference between the average centre of pressure position of a 5-second time period before exposure and the average centre of pressure position of the 5-second time period of exposure. A positive difference signifies a displacement to the right and a negative difference signifies a displacement to the left.
how to calculate frequency bands
On the sway characteristic: coronal velocity
- Using a Fast Fourier Transform with Matlab’s pwelch function
- 5-second time window, overlap of 2.5 s, sampling rate: 10,000
computed the normalized spectrum density and distinguished 3 different bands as described by Paillard and Noe (2015) (0-0.5, 0.5-2, 2-10)
The statistical analysis of the frequency bands was performed using log-transformation of the normalized power density (to ensure a normal distribution)
A back-transformation was performed for representation of the results in tables and charts.
what is permendur?
a cobalt-iron soft magneticalloy with equal parts of cobalt and iron
explain the reasoning for the selection of the frequencies for the study
These specific frequencies were selected for testing on the following basis: 20 Hz was selected because this is the optimal frequency of magnetophosphene perception (Lovsund et al., 1980), 60 Hz is the power line MF frequency in North America, 90 Hz is thought to be the resting hair cell firing rate (Fernandez & Goldberg, 1971; Goldberg & Fernandez, 1971), and 120 and 160 Hz were selected as frequencies above 90 Hz symmetrically from 20 and 60 Hz.
what do we mean by normalized and back-transformed frequency bands?
we used a log-transformation on the normalized spectrum density (y-axis (power) is a percentage of the total power in that frequency band - ie 40% of the power spectrum is occuring in the LFB, 1 being 100)
- did statistical test on log transformed variable
- reported means and graphs, in back-transformed units (involves doing the opposite of the mathematical function you used in the data transformation - For the log transformation, you would back-transform by raising 10 to the power of your number).
describe your statistical plan
what is the sample size calculation?
- see sample size calculation document
- used glover results to obtain, from Kadam and Bahlero n-size calculation paper
- We can understand the concept of “effect size” from day-to-day examples. If the average weight loss following one diet program is 20 kg and following another is 10 kg, the absolute effect size would be 10 kg. Similarly, one can claim that a specific teaching activity brings about a 10% improvement in examination scores. Here 10 kg and 10% are indicators of the claimed effect size.
- Zα, Z is a constant (set by convention according to the accepted α error and whether it is a one-sided or two-sided effect) - 5%, 2-sided (testing for the possibility of the relationship to the mean in both directions)
- For Z1,β,Z is a constant set by convention according to power of the study - 80% power
what does the frequency domain alaysis mean?
A given function or signal can be converted between the time and frequency domains with a pair of mathematical operators called a transform. An example is the Fourier transform, which converts the time function into a sum of sine waves of different frequencies, each of which represents a frequency component. - turns into a power v frequncy graph
In statistics, what does the F mean?
F = Effect Variance (or “Treatment Variance”)/Error Variance
or
F = Between-group Variance/Within-group Variance
- the larger an F-value, the better for you to find a “significant” effect
- p-value then just gives a number to how likely a particular F-value is going to occur, with lower p-values indicating that the probability of obtaining that particular F-value is pretty low
in statistics, what does the p value indicate?
- a p value < 0.05 allows you to reject the null hypothesis
- note: the null hypothesis is a general statement or default position that there is no relationship between two measured phenomena, or no association among groups
- example: a p value of 0.01 means there is a 1% probability that you will mistakenly reject the null hypothesis (produce a false positive - error type 1)
- When we say, for example, we will accept a p<0.05 as significant, we mean that we are ready to accept that the probability that the result is observed due to chance (and NOT due to our intervention) is 5%.
describe what a repeated measures anova is
- used when all members of a random sample are measured under a number of different conditions. As the sample is exposed to each condition in turn, the measurement of the dependent variable is repeated. therefore each aprticipant acts as their own control
- asking the question: is there a difference between any one of these groups that is unlikely to be due to chance?
how was the post hoc analysis performed?
- parawise comparisons obtained, using baroffini coefficient
The Bonferroni correction is an adjustment made to P values when several dependent or independent statistical tests are being performed simultaneously on a single data set. To perform a Bonferroni correction, divide the critical P value (α) by the number of comparisons being made. For example, if 10 hypotheses are being tested, the new critical P value would be α/10. The statistical power of the study is then calculated based on this modified P value.
what is the test of sphericity?
- note we corrected for lack of sphericity with greenhouse geisser
what is the variance (statistics)
-In probability theory and statistics, variance is the expectation of the squared deviation of a random variable from its mean, and it informally measures how far a set of (random) numbers are spread out from their mean.
varience = Sum of Squares (variation) divided by its degrees of freedom
- sum of squares = the sum of (difference of value from mean of that set)^2
what is the degrees of freedom, and error?
- In statistics, the number of degrees of freedom is the number of values in the final calculation of a statistic that are free to vary (seens as n-1)
example: exposure effect in the 2x3x5 ANOVA - between (effect) = number of groups-1 = 2
- within (error) = between x (number of participants-1) = 42
what is the eta^2?
- Eta2 can be defined as the proportion of variance associated with or accounted for by each of the main effects, interactions, and error in an ANOVA study
- a measure of effect size in ANOVAs
- ETA^2 = SSeffect/SStotal = 0.021
- this example value means that 2.1 % of the variance is accounted for by that variable
describe the results of your study
what is statistical power?
- The probability that the test correctly rejects the null hypothesis (H0) when the alternative hypothesis (H1) is true (type 2 error)
- Usually most studies accept a power of 80%. This means that we are accepting that one in five times (that is 20%) we will miss a real difference.
- power = (1 –β)
why did you chose those frequency bands?
- as described by Paillard and Noe (2015) - use the same requency bands
why did you think the induced currents of tACS and MF exposures would be comparable?
MF: A study by Gandhi et al. (2001) calculated the time-varying MF-induced current density values in the brain to be 43.9 mA/m2 on average, with a peak value of 466.4 mA/m2 (modeled on 1.6 mT at 60 Hz).
tACS: Maximum values (closer to the stimulation electrode) were modeled to be 140 mA/m2 for a 2 mA stimulus (Merlet et al., 2013). A study by Laakso et al. (2013) found the current density to be on the order of 40 mA/m2 near the vestibular system.
