Once upon a time DJs, or disc jockeys, as they were called, used to arrive at work with a carrier bag full of singles. They would glance over the playlist, maybe select a track or two before delving into the carrier bag for their own favourite songs.

In most music-driven stations today such a cavalier approach would mean instant dismissal. On today's radio little is played that has not been researched. Radio programmers might still have an atavistic urge to read New Musical Express, Melody Maker and Hot Press, but the really important reading, upon which major decisions are made, are the results of opinion polls, reports from focus groups and research into what people like.

DJs, or jocks, have little input into the choice of music. Before going on air they get a computer print-out listing the tracks in order of play, with the times when the jock will break in. At noon last Monday Long Wave Radio Atlantic 252 played Go Deep by Janet Jackson. This was followed by a gap in the music for the DJ to break in, mainly with the all-important station identification, followed by Billie, Babybird and Lovestation, taking the programme up to 12.18 p.m. The hour ended with Found You by Dodgy. This timetable was available last week.

Format radio, as it is called, is the norm and not just for pop stations. Even commercial classical stations will format the music, ensuring it is the sort the listener wants, that there is a mix and that especially popular pieces are played time and time again.

The serendipity that can still be part of John Kelly on Today FM or some of RTE radio's output is simply too risky for most stations, especially during the day, when the big audiences are available.

John Clarke is responsible for music on RTE's 2FM, still the country's most successful music station. 2FM is not computer-driven he says. Research tends towards gut feeling. There is a playlist, but DJs are given considerable latitude. "2FM is personality-driven. The audience expects certain types of music from particular presenters. Even when Dave Fanning presents the Gerry Ryan show to some extent he brings with him his own type of music," he says.

About two years ago 2FM undertook some research. In Dublin, Galway and Cork hundreds of people sat in ballrooms and listened to snatches from hundreds of tracks and marked up computer cards. "We paid top dollar to be told what we already knew," says Clarke.

But with most music-driven stations, research and more research is designed to keep your finger away from the car radio pre-set buttons. Al Dunne, head of programmes at Atlantic 252, holds a weekly music meeting where the playlist is discussed. DJs attend, but the impression is their contribution is small. At that meeting the new releases that will join the playlist are discussed. Research shows what must be dropped.

Atlantic 252 broadcasts mainly to Britain from Co Meath, though industry sources suggest it might have quite a significant listenership, possibly as high as 9 per cent of the radio audience, especially among young people, in Ireland as well. It is owned by the Luxembourg media giant, CLT, and RTE, which has a 20 per cent stake.

Stations like Atlantic 252 define their audience and then work out how to deliver it to advertisers. In Atlantic's case it is 15 to 24-yearolds. In the past its appeal was broader but the strategy was changed as the market changed. In 1989, when the station first went on air, there were 49 stations in Britain. Today there are 195, forcing stations to find smaller gaps in the market.

The heart of most radio today is a piece of software called Selector, leased to radio stations throughout the world by Radio Computing Services (RCS). Selector, as its name implies, selects and what it selects is music - what should be played at what time.

Selector will ensure that the same tracks are not played during the evening drive time as were played when people were heading to work. It will make sure the station does not play four heavy rock songs in a row or too many women artists.

Some stations programme Selector to count beats per minute so the variety of music played can be fine-tuned to the extent of ensuring a mix of beats and rhythms, mood or energy.

Selector will make sure that if a new Spice Girls single is played an old Spice Girls song will not be played at the same time the next day, in case the listeners think it is the same song.

Al Dunne compares Selector to boxes of cards. In the first box are what Atlantic calls its Power category. This is the five most frequently played tracks at a given time (see panel). They are played once every two and a half hours.

The categories then go downwards: new releases (played every four and a half hours), records which have fallen off the Power list (played every five hours). Other categories are played every 18 hours, while Gold tracks are only played every two days. Selector takes a track out of each virtual box, ensuring it corresponds to the station's music policy and the research results. Sometimes the music comes up automatically as timed by selector, though fears of computer crashes still mean that in most instances the DJ actually pushes the disc into a player.

For Atlantic 252 and stations like Dublin's FM 104, testing is more difficult: because both stations are after a younger audience that listens to new music, there is no time to carefully test a new single so the rather unscientific notion of the "gut instinct" comes into play.

Dunne says this can be "great fun, but it is also scary. At the end of the day we are running a business and we want to get it right".

Radio is still great fun, says Dunne who, like so many other DJs, started with his carrier bag full of records on the pirate stations. At Dublin's 98 FM, the top five tracks will rotate about every five hours. The research methods, however, are almost identical to Atlantic. The only difference is that the polling samples are not 17-year-olds, but 30-something housewives, 98's target daytime audience.

Programme director John Taylor describes how 98 FM does its audience research.

About twice a year 200 people from the target demographics are taken to a venue and played the hook-line of possibly 400 songs. Responses are recorded on computer cards and the result forms the core of the station's music policy.

Every week the audience pulse is taken with 30 songs tested on a representative sample over the phone. That will show when a song is heading towards a "power track", in Atlantic's parlance or a "Hot" record in the argot of 98 FM, or when it should be dropped.

Further qualitative research is periodically undertaken with focus groups.

Graphs indicating the popularity of a song show when its rotation should be increased and as it climbs down the graph when it should go into those categories where it is played less, until it eventually disappears altogether.

Last week Atlantic dropped C'est La Vie by B*Witched, which scored 11 under the Tired Of grading, and Dreams by the Corrs, with 12.

98 FM's station manager, Ken Hutton, points to the theme from Titantic. It peaked for ages, but then research showed listeners were finding it irritating and it was dropped.

Taylor, who learned his craft in the highly competitive radio market of New Zealand, where Auckland alone has 25 stations regulated by nothing more than the marketplace, is aiming at 25 to 45-year-olds, but concentrating more on 25 to 35-year-old females: "They are available and most attractive to the advertisers."

The 30-something female listener wants to hear what she listened to as a teenager, but does not want to be left out of what is happening today. The idea is to play people's favourite songs most of the day. Familiarity in the music mix is the most important element. "If I see people singing along with something on 98 FM, I'm happy," says Hutton.

Taylor is "not in the business of making hits, but of playing what people want". He has to introduce music and then watch for the first indications as to whether he is getting it right or wrong by monitoring the research. He watches to see when something hits the mainstream: 1990s dance and techno is now mainstream; Verve has crossed into mainstream. Taylor is selecting all the time from four decades of music. "We live in an era of disposable music, unfortunately," he says.

Broadcasters are well aware of what people say about format radio. Taylor says that while people call it "musical wallpaper" the music should be seen as a filler in a wider package. Hutton says it is not just about what you play, but how it is put together. Listeners want information, chat. They want the radio to be like a friend. That includes competitions, trivia, as well as the big news stories of the day presented in an appropriate way. People must be able to engage with radio, be able to say to themselves that they could have answered the question put to someone on the line, while driving to work or shaving. DJs, says Taylor, are personality presenters. They are part of the package, they are part of what holds the audience.