Lec 2

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Lec 2
2015-04-15 11:06:12
Lec 2
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  1. What is a longitudinal study design?
    when measuring the visual system systematically
  2. What is a cohort longitudinal design?
    • it follows a sample of people
    • assesses them at diff time intervals
  3. A cross sectional survey is based on
    a sample of population of interest drawn at one point in time
  4. What can longitudinal data be used for?
    Normal variations/range and allow predicitons
  5. WHO MGRS was aimed at
    describing growth of healthy breastfed infants living in good hygiene conditions
  6. What kind of study was the WHO MRGS?
    • longitudinal
    • cross-sectional
  7. What statistical data was generated from the WHO MRGS?
    • percentiles
    • z-score values
  8. The WHO child growth standards depict
    normal human growth under optimal environmental conditions
  9. What are percentiles?
    % of observations (or population) that falls below the value of the variable
  10. Percentiles use a _______ scale
  11. Advantages of percetiles (2)
    • intuitively understandable
    • indicates expected prevalence
  12. Disadvantages of percentiles
    • not comparable across diff measures/groups
    • extreme values are lumped into highest/lowest percentiles
    • not suitable for assessing longitudinal growth status
  13. 80th percentile means
    you score higher than 80% or the population
  14. Percentiles are very ______ specific
  15. What are z-scores?
    The number of SD from the mean when distribution is normal
  16. Z-scores are a ______ scale
  17. 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
  18. Disadvantages of z-scores
    difficult for the public to understand
  19. The conversion table between percentiles and z-scores assumes ______
    normal distribution
  20. The _____ for percentiles is equal to the ________ for z scores
    • median average
    • mean
  21. Some argue that criteria should be based on ________ on associations with ______ and cut points must have _______
    • clinical significance
    • higher health risks
    • a specific purpose
  22. 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
  23. What is data used for?
    to assess the status of individuals or groups and consequently modify the environment to achieve the average criteria
  24. What are z-score calculations based on?
    the smoothed model of the raw data
  25. Define accuracy
    how close a measure value is to the actual value
  26. Define precision
    how close the measured values are to each other
  27. Precision is usually assessed as ____
  28. What statistical test is used to measure precision?
    • the repeated measures analysis of variance
    • ANOVA
  29. What is the degree of accuracy?
    half a unit each side of the unit of measure
  30. What is the degree of accuracy when an instrument measures in 2"s?
  31. When an instrument measures in 1"s, the measured value of 7" is any value in between
    6.5 and 7.5"
  32. What is bias?
    A systemic error which makes all measurements wrong by a certain amount
  33. How may bias be assessed?
    using a Bland Altman analysis
  34. If there is bias, all measured values may be ____
  35. If bias is constant, it can be _____
  36. If the mean in a Bland and Altman plot is 0, then there is _________ bias
  37. The ____ the lines are from each other in the _________ plot, the ____________ the results are
    • wider
    • Bland Altman
    • less repeatable and reliable
  38. The customary definition of an abnormality is a z-score of ____ or a distance of ______
    • 3
    • 3SD
  39. Examples of rulers used in imaging (5)
    • ultrasound
    • fundus photography
    • OCT
    • keratometry
    • MRI
  40. What is an ultrasound?
    a non-invasive imaging technique
  41. Ultrasounds allow the imaging of (3)
    • soft tissues
    • body cavities
    • the eyes
  42. Ultrasounds are often used for
    fetal evaluation during pregnancy
  43. 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
  44. The higher the frequency, the ___ the wavelength
  45. As wavelength shortens, the image resolution of an ultrasound
  46. A ______ relationship exists between ____ and ________ of tissue penetration in an ultrasound
    • direct
    • wavelength
    • depth
  47. The shorter the wavelength the more ________ the penetration in an ultrasound
  48. B-scan is manufactured with _______ frequencies of about _______ oscillations per sec or ____
    • very high
    • 10 million
    • 10MHz
  49. In an A scan, the _______ the echo, the ______ the spike
    • stronger
    • higher
  50. The echoes returned from an A-scan are
    converted into spikes that arise from the baseline
  51. 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
  52. 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
  53. 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
  54. OCT generates
    • cross-sectional images of ocular structures
    • 7-10 micrometre axial resolution
  55. OCT uses ___ and has _____
    • light waves
    • no contact with tissue examined
  56. OCT relies on
    low coherence interferometry to generate images using near-infrared light beams (820nm)
  57. Near infrared in OCT scans allows (2)
    • good tissue penetration
    • reflections from the retina and anterior eye
  58. What is interpreted in an OCT scan to reconstruct an image?
    the magnitude and relative location of backscattered light from the tissues microstructures
  59. Confocal scanning microscopy is
    better than standard light microscopy as it allows visualization deep within living and fixed cells and tissues
  60. What is the res for the HRT?
    • has an axial res of 5-10 micrometers
    • lateral res of 1-2 micrometres
  61. What is the mag of the HRT?
    • ~600x
    • i.e. cornea at a cellular level
  62. The HRT uses a
    • Class 1 laser
    • not harmful to cornea
  63. How does the HRT work?
    • condenser scans across the cornea
    • requires coupling viscous gel/direct applanation of the cornea
  64. MRI produces
    high res images of internal structures
  65. 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
  66. Benefit of MRI
    avoids health risk of x-rays and CT scans