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