# Property NFT rating system
## Property NFT tiers
Each building is categorized into four tiers based on its real-world market value, providing a basis for differentiating its value within Open Meta City. The tiers are subject to change in response to fluctuations in real-world market prices.
### Base Rating
Each building is assigned a base rating based on the "highest actual transaction price in the last five years". The base rating is up to 4 stars, while 5 and 6 stars can be achieved through subsequent market transactions and the DAO building upgrade system.
### Average Price Rating
> Average market price: The sum of the trading price of each NFT at the NFT marketplace, excluding the highest and lowest prices, divided by the number of units.
The average market price rating will line up the apartments based on the average market price, giving the top 0.1% of apartments a 6-star rating, the next tier up, the top 1.2% of apartments, a 5-star rating, and so on. The higher the rating, the more rental income you will receive daily.
* Rating adjustments are automatically reflected at 0:30 daily based on the average price of apartments and NFTs.
* Tiered rental income formula: Internal daily rental income x Tiered weighting
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