Stacker is a popular arcade game designed and manufactured by LAI games and commercialized by entertainment distribution companies[1]. It is a successor of Claw cranes.

The goal of the game is to align rows of moving blocks on top of each other. A player who can stack eleven rows will win a minor prize, which is usually low in value, sometimes lower in value than the amount of money the player paid to play the game. A player who gets to the top row wins a major prize. The major prize varies from machine but will often include game consoles, cellular phones, and MP3 players.

The gameplay is the following. There is a row of three cubes which move side to side on the screen, at the first row. When the player pushes the start/stop button, the row of squares will stop. Then, another row of three moving squares appears above the previous row, moving faster than the one before it. If the squares do not align directly above the previous set, any overhanging squares will be removed. If the player misses completely, the game is over. The number of available squares is automatically reduced to two, then one, during the game. The goal is to consistently get the squares directly above the previous set, "stacking" them to the minor prize and ultimately major prize levels.

In a typical game, what is observed is the following. The player easily manages to get to the minor prize level, refuses it and carries on. At this point three consecutive successful moves lead to winning a major prize. But from now on, the unit to align with the stack is a single block. Usually if the player is focused enough, he achieves to stack the next block and the penultimate one, however stacking the last one fails by a very small fraction. As a result the player loses his coin. And because he was so close to win, he might want to try again, with the same outcome.

The most interesting part is the last step, the one leading in general to a game loss.

Why does the so far very skilled player lose at the last step? Is it the pressure of winning a phone? The answer is: because the last step is different. It looks the same as the previous ones, but it is not.

Pressing the stop button while the last single block is displayed at the correct position does not necessarily leads to a win. In fact most of the times it leads to a loss, and a posteriori the block is displayed mis-aligned with the stack. Only in certain specific cases a win will be obtained.

LAI Games Stacker operator's manual describe all the internal parameters the owner can set to configure the machine. One of these operators is "P10 = Skill Setting (Major price)", whose description is (Default 8) (Adjustable 1 – 10) This option sets the Skill level for players to reach the Major Prize level, as listed in the table below. As this is a skill game the win rate is only the approximate rate for each difficulty setting. Follows then the corresponding options, with P10 = 1 implying approx 1 win in 20 games, whereas P10 = 10 implying approx 1 win in 800 games. This parameter does not affect the displayed game at all, "P08 = Cube Speed" is a different parameter.

In practice, Stacker behaves as the following. For the sake of simplicity, let's imagine that all players are super skilled (they always press the stop button when the last block is correctly aligned), and that P10 = 8 (approx 1 win in 400 games). Instead of having all players win, only 1 over 400 will win. How to achieve this? Once could propose to throw a 400-side dice to decide if a game is a win, but the skill aspect of the game would be instantaneously lost. A more elegant way to do it is to define ''concretely winning states" as a subset of "appearantly winning states", like the following pseudo-code does:

if block is well aligned:
if internal_timer_in nanoseconds % 400 == 0:
proceed to winning
else if stop_button was pressed before block_duration / 2:
move displayed_block to previously_displayed_block
else:
move displayed_block to next_to_be_displayed_block
proceed to losing


This is compatible with purported LAI's statement that Stacker is "100% a game of skill and although it is very difficult, every game played can be a winning game".

However, this theoretically skill game is in practice a luck game, so Stacker is practically a gambling machine. This great video comes to the same conclusion for another arcade game, Cyclone.

Psychologically, having the player close to winning but finally losing is part of the design of this arcade game. Stacker operator's manual describes the machine as a bright and attractive display, simple but exciting game play and a real “Ahh! Just missed” feeling. Indeed the player can see that he was very close, and the belief that the loss is due only to a very tiny amount of bad luck triggers the envy to replay with the confidence to be more accurate next time and win. In reality it can be said that the progressive feelings of self-esteem building with the initial successes, frustration while losing by a near-miss, and hope while deciding to replay, have been entirely engineered by the game designers. You see a game that you control, you live a game who controls you.

Let's conclude with LAI Games' motto: Fun and Profits Go Hand-in hand.

#### Note

[1] Stacker is not commercialized anymore. It has been superceded by Mega stacker, with tips to buyers to improve its performance (i.e. revenue).