To improve your game in StarCraft 2, you must understand a theory
called Opportunity Cost.  It's a little term in economics that
means in order to choose one choice, you may pay a cost in time,
resources, or other options that could have been taken
instead.  Abilities, production, and even build orders have
Opportunity Cost associated with them.  Back in the glory days
of RTS gaming, you had Age of Empires.  There were two terms
for your build order--it was a rush, or it was a boom.  A rush
sacrificed workers for early offense, and a boom sacrificed early
defense for mass production of workers to reach a powerful end game
that much quicker.  Understanding that basic concept of
decision making is essential to understanding the more subtle ones.


Terran Themes

Orbital Command upgrade
- The faster you get your OC up, the
sooner you can start pumping out the lovely MULEs to put your economy
into overdrive.  Timing your build order to have an SCV finish
as your Barracks finishes, and then having at least 200 minerals on
hand (for a Marine or Tech Lab, and the upgrade) is a key part of
refining your early game build order.  This goes for creating
expansions as well.  If you are building your expansion in
your base with the intention of flying it to its destination, you also
have to consider this.  Unless your are moving to an island
expansion where you need to carry SCVs with you, upgrading to an
Orbital Command before positioning your Command Center can get you half
a MULE's worth of energy while en route to its destination. 
This would also enable SCV production the moment it arrives! 
Of course, that point is naught when you realize that you have to spend
that time creating the Orbital Command either way, but it's much safer
to upgrade in your base than in the field.

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Orbital Command energy
- As glorious as the MULE and Scanning
are, and lets say the Supply Drop too, each one costs 50 energy in a
limited pool.  Using one ability limits the ability to use the
others.  And there is nothing worse than forgetting to bring
detection with you and suddenly Dark Templar are cutting up your Siege
Tanks.  You hit your hot key for your Orbital Command, and
instead of hearing Dark Templar explode, you hear Not Enough
Energy.  While this could be averted by saving some energy for
a Scan, saved energy in the Orbital Command is worthless.  You
could mine 270 minerals faster and free with a MULE, or generate 100
free minerals with a Supply Drop if things are looking grim on your
expansions and mineral lines.  It's up to you, the player, to
walk that fine line of use and conservation for a disaster. 
There is one general rule of thumb though--a 50 Mineral SCV is almost
always better as a scout than a 50 energy scan.


Moving out
- When the time is right to move out, you have to make sure
the enemy isn't waiting for you to roll out and leave your base
defenseless.  This goes for any army, but doubly so for Terran
due to their slow units.  Should be losing a fight and decide
to retreat, that often won't be a viable option since your units are so
slow and possibly immobile due to surrounding or Siege Mode. 
So when you move out of your base on the offensive, it must be
decisive.  It must have the edge to, at the very least,
devastate your opponent's army into a retreat.  Even in the
skies, your units will be outrun.  In the end, you need full
intelligence on your opponent's forces to know that you have an
advantage you can press.  Scout early, and scout often so that
you know your attacks will be overpowering and require your opponent's
full attention, rather than having them try to sneak in a pair of Void
Rays while you're in the middle of the field.  Your force must
be devastating enough that those raiders are necessary to defend!


Zerg Themes

Expansion Timing
- Zerg are by far the easiest race to fast expand
with, and that fast expansion can give you the early production and
resource edge you need to win.  However, fast expansions are
also highly vulnerable to rush and harassment tactics.  The
timing of your expansion and Spawning Pool sets the tone for the
game.  If you're unfamiliar with this notation, the number
beforehand is your population in workers before you put down the
building.  These are listed in order of most offensive to
least offensive.


10 Pool - Enables punishment of overzealous economy builds while not
ruining your own

Overpool (Built after your Overlord)

13 Pool

15 Pool - Hatchery is thrown down immediately before or after it to
enable faster double Queen and a boom in production far sooner.


The later your Spawning Pool, the earlier your Hatchery can be put
down.  If your opponent scouts quickly and sees no Spawning
Pool down, they may change their tactics to a Rush oriented
strategy.  You must have solid unit control to do these early
hatchery builds to keep your drones and initial Zerglings alive from
the inevitable harassment or rush, until your Queens can assist in
driving it off.  On the other hand, a powerhouse economy that
isn't rushed inevitably leads to an incredibly potent mid and late
game--just ask Idra, one of the top Zerg players who almost always does
a 15 Pool, 14 hatchery.


Queen Use -
Just like the Orbital Command, Queens must use their energy
very carefully for the best thing possible in the situation. 
Spawn Larvae is your bread and butter ability that should be used on a
Hatchery every time it's up for the most part, unless there are a few
things going on


#1 - Do you have a line of Creep going to your expansion, and a good
ways in front of your expansion?  Do you have sight all around
your base?  Intelligence and Speed are as essential as
production to a Zerg player.

#2 - Can you afford to produce from all of your bases if you're using
Spawn Larvae at each?  Sometimes, before a base is saturated
with workers, you simply cannot afford to keep up full production and
you'll sit at 3 or more larvae, wasting the natural production and as a
result wasting the energy of your queen.

#3 - Are you harassing?  Saving your energy to heal heavily
wounded Mutalisks is like putting another 100 minerals and 100 gas back
in your pocket, and if you're continuing Mutalisk production, you
likely won't need the Spawn Larvae to keep your gas reserves at zero
from production.


Protoss Themes

Chronoboost Use
- From the start, you've got access to boosting any
production or research by 50%.  The first Chronoboost you do
will set the pace for the rest of the game, much like the Zerg timing
of their expansion and Spawning Pool.   Many players
opt to Chronoboost their Nexus once after their first pylon finishes,
and then switch into production Chronoboosting until Warp Gate tech is
available, and then Chronoboost that to completion ASAP. 
Chronoboosting your Nexus more than once can supercharge your economy
early, but requires you to put more minerals into Probe production and
can delay the timing of your Gateway and Cybernetics Core, and possibly
run you into supply cap issues early on.  Chronoboosting
research out is a mixed bag as well--would your army really benefit
more from it than faster production of units?  Take into
account your current income level and threat of the enemy before your
Chronoboost.  If you can't afford your current number of Warp
Gates, there is no point in Chronoboosting the cooldown on them, which
is a solid time to Chronoboost your Forge to complete those valuable
weapons upgrades.

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Hard Counters vs. Hard
- The Robotics units Immortals and
Colossi are absolute powerhouses on the battlefield, but also
incredibly easy to counter.  You have to make a heavy
judgement call when you bring these units into the field of whether or
not they can serve their purpose before they are annihilated by
Zerglings, or anti-air units.  Colossi themselves are a
massive tech investment of at least 700 Gas before one useful one is in
the field (100 Facility, 200 Support Bay, 200 Range Upgrade, 200 for
one colossus).  As such, hiding your tech is incredibly
important so that you have a window of dominance to make something
happen before the hard counters to these monsters hit the field and
quickly destroy them or at least hinder their use. 

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Last Updated: Mar 13, 2016