An attribution model is how you would like to credit conversions in your conversion path in Google Ads. There are 6 different attribution models available to pay per click management services or an individual managing their own account. The 6 different types available are in the image below.
This is a bit of a black box as the data is applied by Google themselves based on your account and how important the clicks in the path were to converting. I would not recommend starting with this attribution model, but rather starting with first click and then switching to data driven once there is sufficient data in the account. This model works well with SMART campaigns.
This applies all the conversion credit to the last advert that the customer clicked, meaning that it tends to apply credit to brand search terms and none to any of the proceeding exploratory clicks. Good pay per click management services will tend not use this attribution model. This is the default option for Google, so this should be one of the first thing you should consider changing in your account to improve performance.
This is the best attribution model to use for growth, as it applies the conversion to the first touch point a customer has had with your business. Which means that it shows you HOW people are finding your business. Good pay per click management services should favour this attribution model.
This applies credit for the conversion across all touchpoints in the conversion path. The trouble with this type of attribution is that ALL touchpoints are considered equal which is not the case in most instances.
This attribute the conversion based on time, for instance the last attribution would get more than a click two weeks ago. Like last click it applies to much credit to the final step in the customers journey and so could lead you to believe that your brand campaigns are more important to your account than they are.
Time decay and the last click attribution could shrink your reach as Google optimises towards your brand campaigns and way from the exploratory search terms which are how your customers are finding you!
Position based attribution applies equal credit to the first and last touchpoints and then splits the rest equally across the other points in the conversion path. This is a bit of a compromise between last & first click, but there could have been more important steps along the way that have been undervalued.
When you change to anything other than first or last click your conversions will be displayed as decimals. At first glance it does not make sense, how can you have half a conversion? But it is simply a way for Google to show you which keywords are working for you and allows Google to optimise your account.