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Audiences might not know about dual-channels theory, but they do know how they feel when presenters don t integrate the concept into their PowerPoint approach. When presenters read bulleted text from the screen, audiences complain that the presenter should E-mail it to me! or Just give me the handout! This frustration has a research basis writing out the text of your presentation on your slides and then reading it to your audience contradicts the widely accepted theory of dual channels. You might assume that presenting the same information in multiple ways will reinforce your point. But if you present the same information to the two channels, you reduce the capacity of working memory and in turn reduce learning by creating what researchers call the redundancy effect. When someone speaks, you process the verbal information at one speed. When the speaker also displays the text of the speech, you process the information at a different speed your mind rst takes in the text visually and then verbalizes it for processing in the verbal channel. Because the same information is arriving through the same channel at different speeds, working memory has to split attention between the two sources of information as it works hard to reconcile them. This redundancy quickly overloads working memory and impairs learning.

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Note On OS X and Linux, you might be able to use a shorter format to use scripts in the script folder,

such as ./script/generate. However, the preceding format is designed to work across platforms by referring to the Ruby interpreter explicitly.

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Until now we ve been working (almost) exclusively with expressions, the ingredients of the recipe But what about statements the instructions In fact, I ve cheated I ve introduced two types of statements already: the print statement, and assignments So, what s the difference between a statement and an expression Well, an expression is something, while a statement does something (or, rather, tells the computer to do something) For example, 2*2 is 4, whereas print 2*2 prints 4 What s the difference, you may ask After all, they behave very similarly Consider the following: >>> 2*2 4 >>> print 2*2 4 As long as you execute this in the interactive interpreter the results are similar, but that is only because the interpreter always prints out the values of all expressions (using the same representation as repr see the section on string representations later in this chapter).

generate has created the db/migrate folder for you (where all migrations are stored) and then created a Ruby file called 001_add_entries_table.rb where you can define what

Note Even though you used the name AddEntriesTable for this migration, it could, in effect, be almost

Redundancy also happens when the same information is presented both visually and in text because the same information is entering through two channels and the mind has to exert more effort to reconcile them. This reduces the ef ciency of working memory and can lead to the cognitive overload that so frustrates audiences. This inef cient duplication of information is unnecessary, because the verbal channel is perfectly capable of understanding the narration without the display of text to the visual channel.

Let s look at 001_add_entries_table.rb:

This is an empty migration featuring two methods, up and down. You use up to create things and perform operations necessary for the migration to work. You use down when rolling back from the state where a migration is finished back to how things were before the migration was run. In the up method you ll use some methods Rails provides to generate the entries table, and in the down method you ll use a method to remove that table. The final 001_add_entries_table.rb looks like this:

That is not true of Python in general Later in this chapter, you ll see how to make programs that run without this interactive prompt, and simply putting an expression such as 2*2 in your program won t do anything interesting4 Putting print 2*2 in there, on the other hand, will in fact print out 4 The difference between statements and expressions may be more obvious when dealing with assignments Because they are not expressions, they have no values that can be printed out by the interactive interpreter: >>> x = 3 >>> As you can see, you get a new prompt immediately Something has changed, however; x is now bound to the value 3 This is a defining quality of statements in general: They change things For example, assignments change variables, and print statements change how your screen looks.

class AddEntriesTable < ActiveRecord::Migration def self.up create_table :entries do |table| table.column :title, :string

Myth: It s OK to read my bullet points from the screen. Truth: Research shows that people understand a multimedia presentation better when the words are presented as verbal narration alone, instead of verbally and as on-screen text.

table.column :content, :text table.column :created_at, :datetime end end def self.down drop_table :entries end end