In developmental psychology the main
goal is to investigate how people stay
the same over the course of their lives
and also how people change over the
course of their lives. And so because we
have this element of aging and of time,
we need to use some different methods
than we would in other fields of psychology.
So, earlier in the semester we
learned about other methods, so, things
like case studies, naturalistic
observation, archival research, surveys,
experiments and so on. And all of these
are still valid in developmental
psychology: It's not that we are getting
rid of these methods,
but we're adding to it,
we're kind of overlaying two
additional methods or two main methods
to help us get at that element of time.
And those are longitudinal and
cross-sectional research. And we're going
to go over each one in a moment, but the
main thing to keep in mind is that those
two new methods are not replacing the
others. Instead, think of it as a "Column A"
and a "Column B." So you can have a
longitudinal case study, a longitudinal
survey, a longitudinal experiment, or you
could have a cross-sectional survey, a
cross-sectional experiment, and so on.
And there are other 
developmental research methods,
but they are all just variations on these two.
So let's start with longitudinal research:
The main idea behind this method is that
you take one group of people who are all the same age
and you study them 
over some chunk of time.
This could be months, it could be years, 
it could be decades.
So the advantage of this is that you're
studying this same change in the same
people over time.
So you get to actually compare 
one person's data at an older age
with their own data from a younger age.
But the downside is that this takes a lot of time
and it takes a lot of money.
And you have the risk
that participants will either die or
they'll just drop out of your study.
And for whatever reason,
maybe they don't want to be in your study anymore,
maybe they move and they don't 
give you the forwarding address,
could be a lot of reasons.
So this is a wonderful method to use,
it gives us a lot of really good
data, but it does have some downsides.
So very often what researchers will do
instead is
what's called a cross sectional study.
And in a cross sectional study, 
you go out at one point in time
and you compare groups of people of
different ages.
So I might go out today
and find a group of 10 year olds, a group
of 20 year olds,
and a group of 30 year olds.
The advantage is everything
that you don't have
with the longitudinal research.
So it's cheaper, it's quicker, 
and you don't have to worry
about people dying or dropping out.
But we do have to worry about what are called cohort effects.
And a cohort effect means that if we see
differences between the people 
in those different age groups
maybe it's age that made the difference,
but maybe it's because those people grew up
under different social and historical
and economic circumstances.
So, for example, if I were to go out today
and I measure attitudes about social media
and I survey people who are age 10, age
20, age 30, and so on up to age 80.
If I find that 10 year olds and 80 year olds
have different attitudes about social media,
maybe age is making the difference,
but it's also very likely that it has to do with the fact that
the 80 year olds spent most of their lives
without the existence of social media.
And for a 10 year old, 
social media has already been there.
So these are our two basic methods 
that we can use for developmental psychology.
If you have any questions, 
please don't hesitate to ask me.
And I hope you all have a wonderful day!
