What does "They also occurred equally" mean?

"The additional signals also occurred equally"? It seems a bit hard to understand.

Context:

The experiment[edit]The experiment involved asking volunteers to respond to a go-signal by pressing an electronic "go" button as quickly as possible.[32] In this experiment the go-signal was represented as a visual stimulus shown on a monitor (e.g. a green light as shown on the picture). The participants' reaction times (RT) were gathered at this stage, in what was described as the "primary response trials".

The primary response trials were then modified, in which 25% of the go-signals were subsequently followed by an additional signal - either a "stop" or "decide" signal. The additional signals occurred after a "signal delay" (SD), a random amount of time up to 2 seconds after the initial go-signal. They also occurred equally, each representing 12.5% of experimental cases. These additional signals were represented by the initial stimulus changing colour (e.g. to either a red or orange light). The other 75% of go-signals were not followed by an additional signal - and was therefore considered the "default" mode of the experiment. The participants' task of responding as quickly as possible to the initial signal (i.e. pressing the "go" button) remained.

More:
http://en.wikipedia.org/wiki/Neuroscience_of_free_will#Manipulating_the_unconscious