Alright, let's dive into the world of psychology and understand the beast that is depression. You're probably familiar with the word - it's been tossed around a lot, hasn't it? But let's see what it really means, who it affects, and when it tends to hit.

Think of these as the ingredients for understanding depression: Prevalence Rate (includes Point Prevalence and Period Prevalence) and Onset Age.

• Point Prevalence: This is the proportion of people in a given population who have depression right now. Imagine it like this: if your entire school is the population, Point Prevalence would be the number of students feeling blue at this very moment.

• Period Prevalence: This looks at the proportion of people who have experienced depression at any time during a specific period. Like, in a 12-month period, it would include people who are currently depressed plus those who develop depression within that year. Lifetime Period Prevalence means those who have experienced depression at least once in their life. So logically, lifetime prevalence will always be greater than 12-month prevalence, and 12-month prevalence is always greater than point prevalence - it's a bit like a set of Russian dolls!

• Onset Age: This is the average age when people in a certain population first develop depression.

The World Health Organization (WHO) predicts that by 2020, depression will be the second leading cause of disability worldwide! It's a big deal and affects people across all walks of life.

Now, this is fascinating! Major Depressive Disorder (MDD) varies across cultures. A study in 2003 found that the lifetime prevalence of MDD ranged from 1% in the Czech Republic to 16.9% in the USA. Wealthy countries generally show higher rates. It's like how people in different countries prefer different music; the prevalence of depression varies too! Several factors can explain this: for example, how bad must the symptoms be for a person in a certain culture to report a mental health issue?