being effective in developing your deep

neural Nets requires that you not only

organize your parameters well but also

your hyper parameters so what are hyper

parameters let's take a look so the

parameters your model are W and B and

there are other things you need to tell

your learning algorithm such as the

learning rate alpha because on we need

to set alpha and that in turn will

determine how your parameters evolve or

maybe the number of iterations of

gradient descent you carry out your

learning algorithm has other you know

numbers that you need to set such as the

number of hidden layers so we call that

capital L or the number of hidden units

right such as zero and one and two and

so on and then you also have the choice

of activation function do you want to

use a rel you or ten age or a sigma

little something especially in the

hidden layers and so all of these things

are things that you need to tell your

learning algorithm and so these are

parameters that control the ultimate

parameters W and B and so we call all of

these things below hyper parameters

because these things like alpha the

learning rate the number of iterations

number of hidden layers and so on these

are all parameters that control W and B

so we call these things hyper parameters

because it is the hyper parameters that

you know somehow determine the final

value of the parameters W and B that you

end up with in fact deep learning has a

lot of different hyper parameters later

in the later course we'll see other

hyper parameters as well such as the

momentum term the mini batch size

various forms of regularization

parameters and so on and if none of

these terms at the bottom make sense yet

don't worry about it we'll talk about

them in the second course because deep

learning has so many hyper parameters in

contrast to earlier errors of machine

learning I'm going to try to be very

consistent in calling the learning rate

alpha a hyper parameter rather than

calling the parameter I think in earlier

eras of machine learning when we didn't

have so many hyper parameters most of us

used to be a bit slow up here and just

call alpha a parameter and technically

alpha is a parameter but is a parameter

that determines the real parameters our

childhood consistent in calling these

things like alpha the number of

iterations and so on hyper parameters so

when you're training a deep net for your

own application you find that there may

be a lot of possible settings for the

hyper parameters that you need to just

try out so apply deep learning today is

a very imperiled process where often you

might have an idea for example you might

have an idea for the best value for the

learning rate you might say well maybe

alpha equals 0.01 I want to try that

then you implemented try it out and then

see how that works and then based on

that outcome you might say you know what

I've changed online I want to increase

the learning rate to 0.05 and so if

you're not sure what's the best value

for the learning ready-to-use you might

try one value of the learning rate alpha

and see their cost function j go down

like this then you might try a larger

value for the learning rate alpha and

see the cost function blow up and

diverge then you might try another

version and see it go down really fast

it's inverse to higher value you might

try another version and see it you know

see the cost function J do that then

I'll be China so the values you might

say okay looks like this the value of

alpha gives me a pretty fast learning

and allows me to converge to a lower

cost function jennice I'm going to use

this value of alpha you saw in a

previous slide that there are a lot of

different hybrid parameters and it turns

out that when you're starting on the new

application I should find it very

difficult to know in advance exactly

what's the best value of the hyper

parameters so what often happen is you

just have to try out many different

values and go around this cycle your

trial some value really try five hidden

layers with this many number of hidden

units implement that see if it works and

then iterate so the title of this slide

is that apply deep learning is very

empirical process and empirical process

is maybe a fancy way of saying you just

have to try a lot of things and see what

works another effect I've seen is that

deep learning today is applied to so

many problems ranging from computer

vision to speech recognition to natural

language processing to a lot of

structured data applications such as

maybe a online advertising or web search

or product recommendations and so on and

what I've seen is that first I've seen

researchers from one discipline any one

of these try to go to a different one

and sometimes the intuitions about hyper

parameters carries over and sometimes it

doesn't so I often advise people

especially when starting on a new

problem to just try out a range of

values and see what works and then mix

course we'll see a systematic way we'll

see some systematic ways for trying out

a range of values all right and second

even if you're working on one

application for a long time you know

maybe you're working on online

advertising as you make progress on the

problem is quite possible there the best

value for the learning rate a number of

hidden units and so on might change so

even if you tune your system to the best

value of hyper parameters to daily as

possible you find that the best value

might change a year from now maybe

because the computer infrastructure I'd

be it you know CPUs or the type of GPU

running on or something has changed but

so maybe one rule of thumb is you know

every now and then maybe every few

months if you're working on a problem

for an extended period of time for many

years just try a few values for the

hyper parameters and double check if

there's a better value for the hyper

parameters and as you do so you slowly

gain intuition as well about the hyper

parameters that work best for your

problems

and I know that this might seem like an

unsatisfying part of deep learning that

you just have to try on all the values

for these hyper parameters but maybe

this is one area where deep learning

research is still advancing and maybe

over time we'll be able to give better

guidance for the best hyper parameters

to use but it's also possible that

because CPUs and GPUs and networks and

data says are all changing and it is

possible that the guidance won't to

converge for some time and you just need

to keep trying out different values and

evaluate them on a hold on

cross-validation set or something and

pick the value that works for your

problems so that was a brief discussion

of hyper parameters in the second course

we'll also give some suggestions for how

to systematically explore the space of

hyper parameters but by now you actually

have pretty much all the tools you need

to do their programming exercise before

you do that adjust or share view one

more set of ideas which is I often ask

what does deep learning have to do the

human brain