```ruby
class CountryStatistic < ActiveRecord::Base
  belongs_to :country

  def national_percentage
    (pa_marine_area / marine_area) * 100
  end

  def total_marine_area
    marine_area + overseas_total_marine_area
  end

  def total_protected_marine_area
    pa_marine_area + overseas_total_protected_marine_area
  end

  def overseas_total_protected_marine_area
    country.children.map(&:statistic).map(&:pa_marine_area).inject(0) do |sum, x|
      sum + (x || 0)
    end
  end

  def overseas_total_marine_area
    country.children.map(&:statistic).map(&:marine_area).inject(0) do |sum, x|
      sum + (x || 0)
    end
  end

  def overseas_percentage
    (overseas_total_protected_marine_area / overseas_total_marine_area) * 100
  end

  [:land, :marine].each do |type|
    field_name = "percentage_pa_#{type}_cover"
    define_singleton_method("global_#{field_name}") do
      stats = where.not(country_id: nil)
      sum =
        stats.map(&field_name.to_sym).inject(0) do |_sum, x|
          _sum + (x || 0)
        end
      (sum / Country.count).round(2)
    end
  end
end
```

Add comment that calculation needs to be confirmed
user
```ruby
class CountryStatistic < ActiveRecord::Base
  belongs_to :country

  def national_percentage
    (pa_marine_area / marine_area) * 100
  end

  def total_marine_area
    marine_area + overseas_total_marine_area
  end

  def total_protected_marine_area
    pa_marine_area + overseas_total_protected_marine_area
  end

  def overseas_total_protected_marine_area
    country.children.map(&:statistic).map(&:pa_marine_area).inject(0) do |sum, x|
      sum + (x || 0)
    end
  end

  def overseas_total_marine_area
    country.children.map(&:statistic).map(&:marine_area).inject(0) do |sum, x|
      sum + (x || 0)
    end
  end

  def overseas_percentage
    (overseas_total_protected_marine_area / overseas_total_marine_area) * 100
  end

  # TODO Need confirmation regarding this calculation
  [:land, :marine].each do |type|
    field_name = "percentage_pa_#{type}_cover"
    define_singleton_method("global_#{field_name}") do
      stats = where.not(country_id: nil)
      sum =
        stats.map(&field_name.to_sym).inject(0) do |_sum, x|
          _sum + (x || 0)
        end
      (sum / Country.count).round(2)
    end
  end
end
```
assistant