Validity and Reliability – Motor Control

When we use a measuring device in the lab to measure something, we need to make sure that our test is valid and reliable.

So what do they mean?

Validity:

Validity means our device measures what we want to measure in a fairly accurate way. How do we know that our device measures precisely? We might have some universal measurement and if our device shows a good correlation with that universal measurement, then we could say proudly that our measuring device is valid  So, what you do is basically compare your measurement/test with another valid test.

Reliability:

Reliability is the repeatability of our test. That is, how consistent is our measurement every time.

Example 1: Supposing our device is a weight measurement device and each time it shows a different weight, say 100 lbs one time and after 10 minutes 150 lbs, can we rely on that measurement? The test-retest is not reliable.

Example 2: Supposing we are using a new-goniometer-kind of device to measure the joint angle for an individual. Person A is recording 50 deg, while person B is recording 70 degrees for the elbow flexion of the same individual. Can we rely on this device? Each person is getting a different output.  The inter-rater reliability is bad.

The picture depicts the targets of an archer. The blue clustered targets show that the archer is way-off from the actual target (in our case, measurement) but is consistent. However, our measurements need to be close to the target in order to be valid. Therefore, reliability does not guarantee that our device measures fine. We need validity so that we can say that our device measures what we actually want to measure.

Sampling Frequency

What is sampling frequency?

When we convert an analog (continuous) signal to a digital (discrete) one, the frequency at which we sample the signal is termed as sampling frequency or sampling rate. How frequent are we sampling is the sampling frequency. Supposing we sample at 1000 cycles  or units per second, then the sampling frequency is 1000 Hz.Either we can mention the digital signal by its sampling frequency or by the sampling interval. Sampling interval is the interval between any two samples. Here in our case (1000 Hz-sampling frequency), the sampling interval will be 1/1000 = .001 (1 millisec).

Finding a threshold for an array – Matlab

To find  a threshold in a one-dimensional array and plot the same in Matlab: Here is the plot you will get after running the program.

a=[1 2 4 6 8 12 15 -18 25 -35 3 4 8 -3 10 33];

b=abs(a);

c=max(b);

m=min(a);

d=.7*c; k=1;

for i=1:size(a,2)

if b(i)>=d

k=i;

break

end

end

plot(a);

line([k,k],[c,m],’Color’,'r’,'LineStyle’,'-’,'LineWidth’,1);

a=[1 2 4 6 8 12 15 -18 25 -35 3 4 8 -3 10 33];    % one-dimensional array

b=abs(a);  % rectifying the data

c=max(b);  % finding out the maximum value

m=min(a);

d=.7*c;    % finding the threshold value, say 70 percent of the maximum value

k=1;

for i=1:size(a,2) % size of the array

if b(i)>=d

k=i;

break

end

end

plot(a);

line([k,k],[c,m],’Color’,'r’,'LineStyle’,'-’,'LineWidth’,1);

Osmosis

Diffusion?

Whenever there is a concentration gradient difference of molecules exists, diffusion occurs. Molecules typically move from an area of higher concentration to lower concentration. For example, when you add a spoonful of salt in a test tube, which is separated by a permeable membrane, the salt diffuses across the water.

Solvent – water

Solute – salt

Net result – diffusion

Example in human body: Oxygen diffuses from an area of high concentration (artery) to an area of low concentration (cell).

Osmosis?

Now, in the above example, supposing if the membrane is selectively permeable , i.e., if the membrane allows only the movement of the solvent (water) and not the solute (salt), then osmosis results. Water will diffuse across the semi-permeable membrane from the low-solute concentration (no-salt) to the high solute concentration (salt), until solvent-equilibrium results. This net diffusion of water occurs passively, i.e., they do not require any energy.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

For example, if you sauté any vegetables with salt, the water oozes out because of osmosis!

Example in plant kingdom: roots from trees suck up the water from soil using osmosis.

So, what happen if you place  cell that is covered by a plasma membrane (which is selectively permeable) in hypo or hyper tonic solutions?

1. If you place a cell in a hypertonic solution (less water & more solutes), the cell will shrink in size.

2. If you place a cell in a hypotonic solution (more water & less solutes – example tap water), the cell will drink up the water & swell!

Check these videos to understand the concept better!

http://www.youtube.com/watch?v=K3wG337xmQc

http://www.youtube.com/watch?v=H6N1IiJTmnc&feature=related

 

 

Arithmetic & geometric series

Arithmetic sequence:

If you take a sequence of numbers, the difference between any two numbers will be constant.

Eg: 2, 4, 6, 8, 10, 12, 14…..

Arithmetic series:

Sum of the above mentioned arithmetic sequence of numbers.

Eg: 2+4+6+8+10

Here is a question for u. How will you calculate the sum of the arithmetic series 2+4+6+8+10?

The formula is s = ((a₁+a_n)/2)*n, where a₁ is the first number, a_n is the last number and n is the number of sequence.

S = ((2+10)/2)*5 = 30

Labview – To read a column from two dimensional array

What if you want to read the second column from the two dimensional array given below. Two for loops – one nested inside will read it. 

Click on the following figure to get a clear view!

What about writing it in c??

 Here is the program.

#include <stdio.h>

int main()

{

int a[3][4];  //allocate space for 2-dimensional array

int i, j, p;

for(j=0;j<3;j++)  // to generate the 2-dimensional table – nested loops

{

for(i=1;i<5;i++)

{

a[j][i]=i;

printf(“%d “, a[j][i]);

}

printf(“\n”);

}

printf(“enter column number: “);  

scanf(“%d”,&p);

for(j=0;j<3;j++)

{

for(i=1;i<5;i++)

{

if (i==p)   // to print the rows in that particular column

{a[j][i]=i;

printf(“%d”, a[j][i]);

}

}

printf(“\n”);

}

}

Doubling Time

The quickest way to remember the doubling time if you know the rate of growth (provided it is constant) is to remember this formula:

T₂ = 70/rate

You can ask me how did I get this. Its simple, just take the use of exponential function.  .7 is nothing but the natural log of 2 to base e.

Example: if you have $10,000 in the bank with a constant rate of growth, say 3%. So, when do you think you could get the double amount?

70/3 = 23.3 years. So, in 23 years whatever you’ve invested will double.

Friction

Supposing you are holding a pen statically. The weight of the pen is acting downward which is opposed equally with an opposite upward force by the friction force (F). The perpendicular force that acts across the surface area of the pen will be the normal force (N). At any given time, the friction force will be the product of the normal force and the coefficient of static friction (µ). i.e., F = N * µ.

If the coefficient of friction is high, like in a rough surface, then the normal force required will be less. If the coefficient of friction is very low, like in silky smooth surface, then the normal force will be high to compensate for the slippage.

To put it in other words, the coefficient of friction will be the ratio of the friction force to the normal force. µ = F/N (parallel force/perpendicular force)

Migration of birds….

It is the winter – The ground is covered with a white carpet of snow. You cannot wander outdoors because of the bone-chilling cold and the freezing water. You sit inside your thermal regulated house, sipping piping hot coffee and lazing around in front of your fireplace. Now tell me, how many of you have wondered about the poor tiny little birds that used to flock your backyards in the spring? Are they still out in the cold or have they gone elsewhere?

Looks like except for a few residential species, most of the birds are vacationing in the warmer south. Can you believe, in North America alone, 5 billion birds annually leave their breeding grounds to go south for winter? So, why do they migrate? It’s for survival – to survive from food scarcity and the harsh winter. Migration is probably the most amazing, inspiring natural phenomenon – Imagine a tiny little bird against the most ferocious elements of nature – it has to overcome the inclement weather, heat exhaustion, fiery wind, and other natural disasters to reach its destination.

Let me show you some of the perilous journeys these birds take. The Arctic terns breed in the arctic summer. They follow the coastline of Africa to reach the Antarctic in November and enjoy the summer there – a total distance of 10,000 miles – they do it twice a year, every year till they perish.

Take the case of barn swallows. Have anyone seen a swallow? It such a teeny-weeny little bird that weighs only a mere 20 g. It nests in the British midlands and go all the way to South Africa, an epic journey of 6000 miles, crossing the formidable Sahara desert that itself stretches around 1000 miles. Of the 5-6 offspring that start the journey, only one will return, a mortality rate of 80%.

Or take the example of bar headed goose. They travel from central Asia to the winter grounds in India – they cross the Himalayas at a height of up to 29,000 feet.

Coming to North America, the tiniest bird that you see in your backyard, the ruby throated humming bird has come from the Yucatan Peninsula crossing the Gulf of Mexico to breed and spend its summer here in the east coast. For that matter, most of the colorful song birds that you see and hear their songs like warblers, thrushes, tanagers, and the birds of prey like hawks, eagles, falcons and ospreys have traveled between 1000 to 8000 miles to breed in the Northern hemisphere. Most of them have wintered in central or south America.

Now that we are in Delaware, can we witness this bird migration first hand? Yes you can. The best place to watch the spring migration of the shore birds is to go to the Delaware Bay in May. Thousands of exhausted shore birds like red knots, sander lings, sandpipers arrive here to feed on the horseshoe crab eggs before continuing their journey to their breeding grounds in the Tundra and Boreal forests of the north.

The best place to watch the fall migration is Cape May. Here you can observe the migratory hawks and millions of songbirds. Here they concentrate in huge flocks to wait for the right weather conditions to cross the Delaware Bay to go south.

Now, next time in spring when you see a colorful bird or hear a heart-rending melodious song, please remember, these tiny little heroes travel thousands of miles every year against all odds just to ‘survive’.

My encounter with elephants….

Before starting my encounter with elephants, let me give you a short introduction about the elephants. Elephants are social animals just like humans. They live in closely knit family groups that are led by the oldest female of the herd. Adult males are rebellious and solitary and associate with the herd only for mating. India holds the largest number of asian elephants – around 25,000, of which southern India is home to around 12,000 elephants. In addition to deforestation, disease and poaching, human-elephant conflict seems to be the most widespread and difficult issue to tackle in elephant conservation. Conflict often occurs when humans move into elephant habitat.

In 2004, when I was working in India, I had a strange request from a scientist friend of mine who works with elephants. He asked me to help him out in ‘elephant census’ in Bannerghatta National Park in southern India about 20 km from Bangalore. So I became a volunteer along with 20 other members for 2 days.

The park spreads around 25, 000 acres. It has hills and valleys with the valley containing moist deciduous forest habitat, while the rest of the park is dominated by dry scrubland and dry deciduous areas. For each of the volunteer 2 forest guards were assigned. I carried a binocular & a camera, while the forest guards carried a small drum, walkie-talkie, the maps, food and water, and two loaded rifles. That was the first time in my life I have actually touched the rifle. I asked naively ‘do u ever use the rifle? for which the foreste guard answered that just one month before a forest guard was trampled by a solitary elephant despite of having a rifle. The gravity of the situation was slowly sinking to me. I got the bottom line message that ‘u don’t take it easy with elephants’.

And so we set off for the elephant census.

The temperature was hovering around in the 90s. We were basically targeting water bodies and ponds where the elephant loves to hang out because of the heat. We were very particular in not going too near them to precipitate an anxiety attack from them. They are highly territorial. Anytime we came across a thick bamboo bush, the guards bellowed loudly at times beating even the drum when they were not quite sure what was lurking behind the bushes. Besides elephants, the Bannerghatta national park is also home to a wide variety of wild animals and reptiles like poisonous snake and scorpions.

We counted atleast five herds for the first day. We photographed the dung, and the footprint of the elephants. It was a productive and tiring day. One of the guards spotted a small honeycomb on our way back and he made a bold attempt to grab it from the few bees that were hovering around it. We plucked few broad leaves and used them as our container while the forest guard poked the honeycomb with a twig to extract the honey. Needless to say, that was the best fresh honey that I’ve ever tasted in my life!!

The next day we were not that lucky to spot a single herd. It was almost late afternoon, when we approached a valley. One of the range guards climbed a tree and spotted a herd at the bottom of the valley, wallowing in a flowing stream. Since we were not sure about the number of elephants in the herd we thought we could approach them a little closer. It was a difficult climb downhill with thorny bushes and no paths. We had to be careful about the reptiles too. After climbing down almost half of the distance one of the rangers told us to pause suddenly. He stood straight straining to catch some sound. There was an eerie silence. All I could hear was the gentle rustling of the trees and the shrubs and my own heavy breathing. Then I realized something was wrong. The ranger guard who was in front of me saw some 5-6 elephants that were coming towards us with their trunks lifted high and were trying to sniff our scents.

Thatz when I saw some 30 feet away a glimpse of a brown mass. I felt rooted to that place just like the tree… and that split second the guard grabbed my hand and started to run down hill on the other direction coz the wind will be in the opposite direction if we run downhill. I don’t know where we had that super human strength. We were bolting like winds and I couldn’t cope up with the speed of the guards where I stumbled and fell twice. That guy never left his grip he just dragged me till we reached the bottom of the valley. It was the mother of all runs. After reaching the valley the forest guard pulled out his rifle and fired a shot in the air. A deafening boom. My knees were shivering and my whole body was covered with dust, bruises and scratches.

By then the guards explained to me that probably that chase was a mock charge because calves were there in the herd. If the elephant were to chase us really they would have got us within no time.

Till this day this incident remains one of the most memorable events in my life….

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