The problem lies in our ability to recall cricket performance. We think we remember things as they happened, but this is rarely the case. Nobody has the memory capacity to remember every ball of every game and practice. You may lose something important, especially around emotionally charged moments: a stunning victory or controversial umpiring decision.
Not even the best coaches have the ability to recall everything, so relying on coaches (or team mates) eyes is also a potential failure.
If you have the wrong feedback you are likely to work on the wrong thing: For example, a technical error that is more of a mental issue. We all know that improvements in any skill take lots of practice. If that practice is misdirected you are wasting your precious time.
The Hawkeye system has given international cricketers the ability to have a record of every ball bowled or faced during a match and often in practice too. It's become an objective replacement for the subjective memory of top level coaches.
But you don't need the expensive technology of Hawkeye to keep a record of performance to analyse later. You can start with a pen and paper. This is called notational analysis.
Noting things down as they happen allow you to go back over your innings or bowling spell at a later date with access to all the information rather than the bits you remember. You can spot general trends in your game that can be worked on in practice.
A player goes to a coach saying he is getting out LBW a lot. The coach consults the scorebook and finds that the player has been out LBW 3 times in 12 innings. In the same season he has also been out caught 8 times. The player has merely remembered the LBWs more clearly. The coach could have gone to work on the player's technique in one way when there is a bigger problem elsewhere.
It's possible to keep a record both in training and in matches. The first question to ask is this: What do you want to know? By having an ultimate goal you only record the information that is useful.
So a bowler who wants to improve their bowling accuracy might ask a coach, scorer or 12th man to record the line and length of each ball in their spell. This could then be converted into a percentage of balls that were accurate. The player can then train to improve this accuracy and see if it makes a difference over the weeks.
Another example could be a captain looking at his field placings. He could have each field recorded and compare it to the score and conditions to see if his tendency is too attacking, just right or too defensive.
Notation is not a perfect system. Firstly, you need someone to record the information. Some teams can find it hard enough to find scorers and umpires so finding someone to take notes during a match can be tough. Players can notate each other but depending on the system you use it may not be accurate. Secondly you need to look through the data that is recorded: something that can be time consuming, especially if you have a complicated system.
If you have access you can use some tools to help you.
Technical analysis is much easier if you can video yourself. You can then go over your batting or bowling technique comparing it to the textbook method. If you have access to a laptop you can record information directly into is so the sorting of the data is automated.
The PitchVision sensor system also takes the hard work out of recording and analysing data. It records your bowling and batting statistics in the nets allowing you to track performance over time. You can see what coaching drills are working.
It's important to remember that notation (computerised or manual) is only as good as the person interpreting the information. A great coach has the ability to identify errors through a combination of notation and the naked eye. They then have the drills and knowledge to be able to correct the error quickly. No recording system can do that for you.
However as a method to coach yourself or others, notation is an essential tool.