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What is a longitudinal study design?
when measuring the visual system systematically
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What is a cohort longitudinal design?
- it follows a sample of people
- assesses them at diff time intervals
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A cross sectional survey is based on
a sample of population of interest drawn at one point in time
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What can longitudinal data be used for?
Normal variations/range and allow predicitons
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WHO MGRS was aimed at
describing growth of healthy breastfed infants living in good hygiene conditions
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What kind of study was the WHO MRGS?
- longitudinal
- cross-sectional
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What statistical data was generated from the WHO MRGS?
- percentiles
- z-score values
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The WHO child growth standards depict
normal human growth under optimal environmental conditions
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What are percentiles?
% of observations (or population) that falls below the value of the variable
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Percentiles use a _______ scale
rank
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Advantages of percetiles (2)
- intuitively understandable
- indicates expected prevalence
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Disadvantages of percentiles
- not comparable across diff measures/groups
- extreme values are lumped into highest/lowest percentiles
- not suitable for assessing longitudinal growth status
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80th percentile means
you score higher than 80% or the population
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Percentiles are very ______ specific
population
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What are z-scores?
The number of SD from the mean when distribution is normal
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Z-scores are a ______ scale
continuous
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Advantages of z-scores (3)
- allows comparison across ages and sexes as it is dimensionless
- able to quantify extreme values
- good for assessing longitudinal change in growth status
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Disadvantages of z-scores
difficult for the public to understand
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The conversion table between percentiles and z-scores assumes ______
normal distribution
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The _____ for percentiles is equal to the ________ for z scores
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Some argue that criteria should be based on ________ on associations with ______ and cut points must have _______
- clinical significance
- higher health risks
- a specific purpose
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It is difficult to base criteria on clinical significance as we need to consider (3)
- short-term health outcomes in childhood
- intermediate- " during adolescence
- long- " during adulthood
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What is data used for?
to assess the status of individuals or groups and consequently modify the environment to achieve the average criteria
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What are z-score calculations based on?
the smoothed model of the raw data
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Define accuracy
how close a measure value is to the actual value
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Define precision
how close the measured values are to each other
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Precision is usually assessed as ____
repeatability
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What statistical test is used to measure precision?
- the repeated measures analysis of variance
- ANOVA
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What is the degree of accuracy?
half a unit each side of the unit of measure
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What is the degree of accuracy when an instrument measures in 2"s?
1"
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When an instrument measures in 1"s, the measured value of 7" is any value in between
6.5 and 7.5"
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What is bias?
A systemic error which makes all measurements wrong by a certain amount
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How may bias be assessed?
using a Bland Altman analysis
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If there is bias, all measured values may be ____
wrong
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If bias is constant, it can be _____
compensated
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If the mean in a Bland and Altman plot is 0, then there is _________ bias
no
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The ____ the lines are from each other in the _________ plot, the ____________ the results are
- wider
- Bland Altman
- less repeatable and reliable
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The customary definition of an abnormality is a z-score of ____ or a distance of ______
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Examples of rulers used in imaging (5)
- ultrasound
- fundus photography
- OCT
- keratometry
- MRI
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What is an ultrasound?
a non-invasive imaging technique
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Ultrasounds allow the imaging of (3)
- soft tissues
- body cavities
- the eyes
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Ultrasounds are often used for
fetal evaluation during pregnancy
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How does an ultrasound work? (4)
- sound waves of high frequency (>20k Hz/sec) are bounced off tissues
- reflected into probe
- echoes are converted electrical signal
- reconstructed into a picture - sonogram
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The higher the frequency, the ___ the wavelength
shorter
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As wavelength shortens, the image resolution of an ultrasound
improves
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A ______ relationship exists between ____ and ________ of tissue penetration in an ultrasound
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The shorter the wavelength the more ________ the penetration in an ultrasound
shallow
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B-scan is manufactured with _______ frequencies of about _______ oscillations per sec or ____
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In an A scan, the _______ the echo, the ______ the spike
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The echoes returned from an A-scan are
converted into spikes that arise from the baseline
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The amplitude in an A-scan is determined by _____ and gives the examiner info regarding ____
- the strength of the echo
- the density of the tissue
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In a B-scan, the brightness is determined by ___ and allows the examiner to determine ____
- the strength of the echo
- the density of the tissue
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Are measurements from fundus cameras accurate? (4)
- no
- photo size is affected by the px's refractive error
- percentage change in sizes may be more reliable
- use a correction factor/normative database for greater accuracy
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OCT generates
- cross-sectional images of ocular structures
- 7-10 micrometre axial resolution
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OCT uses ___ and has _____
- light waves
- no contact with tissue examined
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OCT relies on
low coherence interferometry to generate images using near-infrared light beams (820nm)
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Near infrared in OCT scans allows (2)
- good tissue penetration
- reflections from the retina and anterior eye
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What is interpreted in an OCT scan to reconstruct an image?
the magnitude and relative location of backscattered light from the tissues microstructures
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Confocal scanning microscopy is
better than standard light microscopy as it allows visualization deep within living and fixed cells and tissues
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What is the res for the HRT?
- has an axial res of 5-10 micrometers
- lateral res of 1-2 micrometres
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What is the mag of the HRT?
- ~600x
- i.e. cornea at a cellular level
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The HRT uses a
- Class 1 laser
- not harmful to cornea
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How does the HRT work?
- condenser scans across the cornea
- requires coupling viscous gel/direct applanation of the cornea
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MRI produces
high res images of internal structures
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How does MRI work?
- radiofrequenct pulse of given frequency is applied
- nuclear spins are knocked off axis
- magnitude of signal and decay depends on density of nuclei and their local structural env --> diff for diff tissues --> contrast images
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Benefit of MRI
avoids health risk of x-rays and CT scans
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