Medicine - Measurement and Data Reliability

Question 1

Name 5 steps in drug analysis

Answer 1

1) Take representative sample
2) Extract analyte(s) from interfering matrix
3) Separate analytes
4) Identify, detect and quantify analytes
5) Determine reliability/validity of results

Question 2

5 Steps of analysing a sample

Answer 2

1) Select representative sample
2) Extraction -e.g. a lot of matrix is present
3) Purification - e.g. is the test highly susceptible to impurities, excipients, degradation products
4) Characterisation - the target analyte should be identified, especially if the test is not very specific
5) Data validity - us stats, comparison with standards and/or another method. If you are testing by a newly developed method or if the method has been changed significantly then it is critical to validate the new method first

Question 3

Ways to present and handle analytical data

Answer 3

• Can you group it? - frequency table - histogram if numerical or bar chart if numbers and text
• Did you change only one parameter? Line graph suitable where x axis having independent variable (what you altered) and y axis being dependent variable (what you measured)
• If you changed more than one variable then a scattergram may be suitable

Question 4

How to get the line of best fit?

Answer 4

Linear regression analysis - better than manual determination 
equation of the line of best fit 
y = a + bx 
Use equation where:
a and b are parameters of the line 
b is the slope, a is the y intercept 
N= number of pairs of values

Question 5

Analytical measurements are not exact. What do we do in reaction to this?

Answer 5

Assume the last digit quoted in a measured value is uncertain

Question 6

An analyst weighs 25 tablets and finds the average weight per tablet.
Then he calculates that 38 tablets should weigh 44.2239672g. The accuracy of the balance is +/-0.1mg
How should he write the result?

Answer 6

As Weight= 44.2240g

Question 7

When calculating drug content, how many significant figures is favoured for the answer?

Answer 7

3 significant figures

Question 8

Rules for sigfigs when adding or subtracting?

Answer 8

Round off to the same number of decimal places as the value having the fewest decimal places
e.g.
10 + 28.621 = 129

Question 9

Rules for sigfigs when multiplying or dividing?

Answer 9

Round off to have the same number of sig figs as the value with the least significant figures

e. g.
2. 1 x 11.31 = 24

Question 10

How should one take measurement on a scaled instrument?

Answer 10

Go for one more figure than you can read with certainty
e.g.
Manufacturers often give accuracy value to their apparatus. A graduated 50ml burette may be accurate to only +/-0.02mL
Therefore when measuring 30ml of solvent you should report to this level of accuracy only as 30.00ml

Question 11

What 3 types of errors affect validity of analytical measurement?

Answer 11

1) indeterminate (random) error
2) determinate (systematic) error
3) Gross error

Question 12

What is indeterminate error and what does it affect?

Answer 12

Random error
Evaluate with statistics
affects precision more than accuracy

Question 13

what is determinate error? how to evaluate it?

Answer 13

Systematic error
evaluate with standards
affects accuracy more than precision

Question 14

Gross error is what?

Answer 14

A big mistake, like spilling everything on the floor, sample evaporated etc. the value stands out as an outlier.

Question 15

What do random errors follow statistics wise?

Answer 15

The normal distributions or Gaussian pattern

Question 16

With the normal distribution, use standard deviations to explain how sure one can be where the values are

Answer 16

68% values within one sd (or 68% chance that random error will not exceed +/-1sd)
95% are within +/- 2sd
99.7% are within +/- 3sd
of the mean value

Question 17

What is precision?

Answer 17

How close (repeatable) the measurements are to each other

Question 18

How is precision expressed?

Answer 18

Range (quickest) - difference between highest and lowest results
Standard deviation or the relative standard deviation - preferred. Calculate by statistical analysis

Question 19

What is relative standard deviation?

Answer 19

The spread of data expressed as % of the mean value, so units become unimportant
+/- RSD also called the coefficient of variation (CV)
use RSD to remove units
divide by mean

Question 20

What is accuracy?

Answer 20

how close the measurement is to the accepted value

Question 21

How can we evaluate accuracy?

Answer 21

by calculating the absolute error (measured value minus a true value)
and relative error

Question 22

Equation for absolute error

Answer 22

E=xi-xt

where xi= observed value and xt= true value

Question 23

Equation for relative error

Answer 23

E=(xi-xt)/xt * 100%

percentage value

Question 24

Name and describe 3 origins of systematic error

Answer 24

1) Instrumental
Calibration - apparatus (cylinders, dispensers etc) or equipment (balances, pH meter etc)
2) Method
Reagents (unstable), reactions (incomplete)
3) Personal error
e.g. eye sight (colour), height (can not read burette at eye level) etc

Question 25

What can a null hypothesis that assumes to bias verify?

Answer 25

If a difference is due to a random or systematic error (bias) in the method or not

Question 26

When do you reject the null hypothesis?

Answer 26

If xbar-xt>+/-ts/SRN 
where xbar = mean
xt = true value 
SRN = square root of N 
ts = standard deviation

Question 27

When do you accept the null hypothesis?

Answer 27

xbar-xt

Question 28

What is t-Test?

Answer 28

One of the most powerful and widely used statistical tests
It is used to calculate the confidence intervals of a measurement when the population standard deviation is not known, which is the case with limited number of data points

Question 29

What can the t test be used to do?

What does the t test correct for?

Answer 29

Used to compare two averages

Corrects for the uncertainty of the sample standard deviation caused by taking a small number of samples

Question 30

How to detect gross error?

Answer 30

use a Q test
You can use a Q distribution table to compare the Q experimental value at a given confidence level with Q critical value in the table

Question 31

Using the Q test, when is a value an outlier?

Answer 31

When Qexp>Qcritical

Question 32

Qexp=?

Answer 32

Qexp=(xq-xn)/w
where xq=value in doubt
xn=nearest value to doubted
w=range

Question 33

What is method validation?

Answer 33

The process of demonstrating that analytical procedures are suitable for their intended use and that they support the identity, strength, quality, purity and potency of the drug substances and drug products

Question 34

In method validation, in addition to the details of how to carry out an assay (SOP) what else must the method state?

Answer 34

How the sample is extracted/prepared or handled before it is analysed. Quality of reagents including reference or standard materials used. Procedures for instrument calibration, reagent preparation, method of sample preperation and handling

Question 35

When do methods require complete validation?

Answer 35

When they are new methods

Question 36

When are pharmacopoeial methods not valid?

Answer 36

If they are significantly changed. Any changes must be validated (partial validation)

Question 37

What is the most difficult to validate?

Answer 37

Biological assays. Therefore they are best left alone as they are in monographs as they are very variable.