# Normal or Gaussian Distribution

## Characteristics of Normal Distribution

– Tails should meet the axis at infinity

– Bell-shaped distribution

– Mean = mode = median

– The area under the curve is 1 standard deviation away from the mean and makes up 68% of the entire distribution under the curve (This means that if you randomly select a point under the curve, there is a 68% chance it will fall one standard deviation from the mean)

– The area under the curve 1.96 SD (round to two) away from the mean makes up 95% of the entire distribution under the curve (This means that if you randomly select a point under the curve, there is a 95% chance it will fall 2 standard deviations from the mean)

– The sample mean = mean of the population

– The standard deviation of the mean distribution or standard error = (SD of the population)/(square root of the number of scores)

– The standard error indicates the degree to which sample means deviate from the mean

– The sample mean distribution converges to normal distribution as the size of the sample increases

– The bell-shaped curve can also be reflected in the lay-out of a histrogram

Here the SD is 15 units

## Questions Dealing with Standard Deviation

Question: Assume the standard deviation is 10 and the mean score is 100. If you randomly select any point 1 standard deviation from the mean, what would be your range?

Answer: The range would be between 90 and 110. As one standard deviation is 10 units left or right. You could also say that you have a 68% chance of randomly picking a score between 90 and 110 on the this graph.

Question: Assume the standard deviation is 10 and the mean score is 100. If you randomly select any point 2 standard deviations from the mean, what would your range be?

Answer: The range would be between 80 and 120. As one standard deviation is 10 units left or right, 2 standard deviations would be 20 units left or right. You could also say that you have a 95% of randomly picking a score between 80 and 120 on this graph.

N.B: 95% is the commonly accepted probability, which is the alpha level or confidence level in psychological studies for rejecting the null hypothesis is p<0.05.

## The z-Score

It is possible to convert all normal distributions to the standard normal distribution.

For a standard normal distribution the mean has to equal 0 and the SD has to equal 1.

You can find the z-score by subtracting the mean from each data point, and then dividing the this zero-meaned data by the standard deviation.

If your final data point is +1, this point is one standard deviation above the mean. If your final data point is -3, this point is 3 standard deviations below the mean. The z-score is particularly useful for comparing data across different situations.

## Error Bar Charts

Error bar charts are away of representing the confidence interval. Error bars display your mean means as a point on a chart and a vertical line through the mean point that represents the confidence interval. The longer the line, the longer the confidence interval. Error bar charts can also be used to see if two population means differ from each other by comparing confidence interval. If the confidence intervals do not overlap we can be 95% confident that both population means fall within the intervals indicated and therefore do not overlap.

Bibliography

ZHENG, Y. (2013). Referencing and citation – Harvard style, from PSY104 Methods and Reasoning for Psychologists. University of Sheffield, Richard Roberts Building on 11th February. Available from: Blackboard.
[Accessed 4/02/13].

# Methods Used for Studying Infants’ Perception

Part of getting onto a good masters or Ph.D programme means having real-life experience. As only a second year undergraduate that can sometimes seem like an age away, but time really does fly by. In order to get some experience in research I transcribed videos for a developmental researcher at my department. Even though my job was pretty menial in the whole scale of things, writing down all the speech and movements of infants really made me appreciate something substantial; infants are very hard to understand and observe. Their intentions, their desires and even just their knowledge can be difficult to interpret. As such, psychologists use a set of methods to study infant perception, intentions, desires and capabilities.

This post will deal with studying infant perception.

Preference Technique

Basic set-up

1. A researcher presents two stimuli to an infant simultaneously

2. The researcher monitors the infant’s eye movement. Researchers use various techniques for this, one being the ASL Model 504.

3. If the infant looks more at one stimulus than the other, it is inferred that the infant prefers that stimulus over the other.

If accurate, measures of the eye movements can be made, this technique is quite simple and effective. The infants preference can be inferred because of habituation, a fancy word for boredom.

Habituation

Habituation and dishabituation are another method used to study infant perception and preference. After looking at a stimulus for a certain amount of time, we become bored of it. Just like after awhile we stop feeling the clothes on our body. Our brain gets bored with the touch sensation, and so eventually it stops informing us of it. On this basis, psychologists infer that babies will stop looking at a stimulus if they gets bored of it. If a stimulus is then presented with a new stimulus, it is likely he or she will prefer looking at the new stimulus that the infant has not seen before. If the infant does prefer the new stimulus, we can infer that the infant is capable of discriminating between the two stimuli. Discrimination between two stimuli allows researchers to detect the stage of perceptual development of infant has reached.

Conditioning

Classical and operational conditioning are terms you should be familiar with have you ever taken an introductory psychology course. Conditioning with infants consists of the same learning system. Fortunately, infant studies usually just involve rewarding the infant with pleasant sounds or images, usually of or from their mother.

Basic set-up

1. Infant is given a dummy or pacifier

2. Researcher waits for the infant to begin sucking on it at their usual rate

3. If the infant begins sucking at a faster rate than usual they are rewarded with the sound of their mothers voice

4. The infant will soon learn that as long as her or she continues sucking at the increased rate, they will hear their mother’s voice

5. After awhile, habituation sets in as the baby loses interest in the sound and their sucking rate decreases

6. The researcher then proceeds to introduce a new sound

7. If the infant is capable of discriminating the new sounds, they will begin to suck more again to her this new sound

All of these various tests of perception, as mentioned above are used to measure the development of infants.