Correlation
* Describes the idea of relationships between variables.
* allow expressing relations between signals within a net.
* are connections between different events
  - statistics
  - the most often used statistics in psychology
* break down complex relationships into simpler components.
* concept similar to cosine.
* deals with interconnection of subsystems and key to understanding complex systems.
* essentially represents how much a stock or fund is like another fund.
* exist between local temperature changes and life history phenology.
* increase with biological relationship, even when pair members are reared apart.
* is made between structural arrangements of tissues and functions
  - reciprocity
  - simply a measure of how strong a relationship exists between two variables
* is the basic statistical measure of the relation between two variables
  - engine under the hood of many market-research methods
  - square root of the R-squared value expressed in decimal form
  - used as the fitness measure to drive the evolutionary process
  - when one action or phenomenon causes or is causally linked to another
* measure how variables or rank orders are related.
* measure of the degree of linear relatedness of two variables
  - strength of the relationship between two variables
* measure the strength of a linear relationship between two variables
  - linear association between two variables
* numerical expression that signifies the relationship between two variables.
* occurs when the sampling of one or more input distributions are related.
* plays a major role in factor analysis
  - an important part in evolution
* quantifies how closely two variables are connected.
* refer to relationships between two things - two factors or variables.
  * usually has one of two directions. These are positive or negative. If it is positive, then the two sets go up together. If it is negative, then one goes up while the other goes down.
* refers to the degree of relationship among variables.
* simple measure of how two variables are related.
* simply means that a connection exists between two sets of data
  - relationship of some kind exists between two variables

### correlational statistics:

Negative correlation
* are correlations.
* correlational statistics
* means that as one set of data increases, the other decreases.

Positive correlation
- useful for finding products which tend to be purchased together
* exist between growth and blood concentrations of testosterone.
* is when both variables rise or fall together.
* says that as one set of data increases, so does the other.

### correspondence:

Postcard
* Many postcards are unique works of art.
* are a means by which to express a few sentiments
  - generally black and white photo-postcards
  - much more distinctive than other forms of direct mail
  - single cards sent without a wrapper or an envelope
* includes chapters
  - letters
  - paragraphs
  - sections
  - word order
* is located in fridges
  - mail boxs
  - post offices
  - shops

Corrosive
* Many corrosives are also water reactive.
* Most corrosives are either acids or bases.
* Some corrosives are also flammable or combustible and can easily catch fire and burn or explode
  - incompatible with other chemicals
  - toxic and can cause other health problems
* are materials that can attack and chemically destroy body tissues on contact
  - chemically destroy exposed body tissues
  - cause tissue destruction on contact
* can also damage or destroy metal
  - even destroy metal
  - burn and destroy body tissues on contact
  - destroy containers made of improper materials
* touching the skin can severely irritate or even badly burn and blister the skin.